Unitary graphene layer or graphene single crystal

ABSTRACT

A unitary graphene layer or graphene single crystal containing closely packed and chemically bonded parallel graphene planes having an inter-graphene plane spacing of 0.335 to 0.40 nm and an oxygen content of 0.01% to 10% by weight, which unitary graphene layer or graphene single crystal is obtained from heat-treating a graphene oxide gel at a temperature higher than 100° C., wherein the average mis-orientation angle between two graphene planes is less than 10 degrees, more typically less than 5 degrees. The molecules in the graphene oxide gel, upon drying and heat-treating, are chemically interconnected and integrated into a unitary graphene entity containing no discrete graphite flake or graphene platelet. This graphene monolith exhibits a combination of exceptional thermal conductivity, electrical conductivity, mechanical strength, surface smoothness, surface hardness, and scratch resistance unmatched by any thin-film material of comparable thickness range.

The present invention claims the benefits of the following co-pendingpatent applications:

A. Zhamu, et al., “Graphene Oxide Gel Bonded Graphene Composite Filmsand Processes for Producing Same,” U.S. patent application Ser. No.13/385,813 (Mar. 8, 2012).

A. Zhamu, et al., “Graphene Oxide-Coated Graphitic Foil and Processesfor Producing Same,” U.S. patent application Ser. No. 13/694,161 (Nov.2, 2012).

A. Zhamu, et al., “Thermal Management System Containing a GrapheneOxide-Coated Graphitic Foil Laminate for Electronic Device Application,”U.S. patent application Ser. No. 13/694,162 (Nov. 2, 2012).

FIELD OF THE INVENTION

The present invention relates generally to the field of graphiticmaterials for heat dissipation applications, and more particularly to agraphene oxide-derived graphene monolith or graphene single crystal thatexhibits a combination of an exceptionally high thermal conductivity,high electrical conductivity, high mechanical strength, good surfacescratch resistance, and good hardness.

BACKGROUND OF TILE INVENTION

Carbon is known to have five unique crystalline structures, includingdiamond, fullerene (0-D nano graphitic material), carbon nano-tube (1-Dnano graphitic material), graphene (2-D nano graphitic material), andgraphite (3-D graphitic material).

The carbon nano-tube (CNT) refers to a tubular structure grown with asingle wall or multi-wall. Carbon nano-tubes have a diameter on theorder of a few nanometers to a few hundred nanometers. Its longitudinal,hollow structure imparts unique mechanical, electrical and chemicalproperties to the material. CNT is a 1-D (one-dimensional) nano carbonor 1-D nano graphite material.

Bulk natural flake graphite is a 3-D graphitic material with eachparticle being composed of multiple grains (or graphite single crystalsor crystallites) with grain boundaries (amorphous or defect zones)demarcating neighboring graphite single crystals. Each grain is composedof multiple graphene planes oriented parallel to one another. A grapheneplane in a graphite crystallite is composed of carbon atoms occupying atwo-dimensional, hexagonal lattice. In a given grain or single crystal,the graphene planes are stacked and bonded via van der Waal forces inthe crystallographic c-direction (perpendicular to the graphene plane orbasal plane). Although all the graphene planes in one grain are parallelto one another, typically the graphene planes in one grain and thegraphene planes in an adjacent grain are different in orientation. Inother words, the orientations of the various grains in a graphiteparticle typically differ from one grain to another.

A graphite single crystal (crystallite) is anisotropic with a propertymeasured along a direction in the basal plane (crystallographic a or bdirection) being dramatically different than if measured along thecrystallographic c-direction (thickness direction). For instance, thethermal conductivity of a graphite single crystal can be up toapproximately 1,920 W/mK (theoretical) or 1,800 W/mK (experimental) inthe basal plane (crystallographic a- and b-axis directions), but thatalong the crystallographic c-axis direction is less than 10 W/mK(typically less than 5 W/mK). Consequently, a natural graphite particlecomposed of multiple grains of different orientations has a propertylying between these two extremes. It would be highly desirable in manyapplications to produce a bulk graphite particle (containing single ormultiple grains) having sufficiently large dimensions and having allgraphene planes being essentially parallel to one another along onedesired direction. For instance, it is highly desirable to have one bulkgraphite particle (e.g. a unitary layer entity of multiple grapheneplanes) with all the graphene planes being substantially parallel to oneanother) and this unitary layer entity has a sufficiently largelength/width for a particular application (e.g. >5 cm² for use as aheat-spreading sheet on a CPU of a smart phone). It has not beenpossible to produce this type of large unitary graphene entity fromexisting natural or synthetic graphite particles.

The constituent graphene planes of a graphite crystallite can beextracted or isolated from a graphite crystallite to form individualgraphene sheets of carbon atoms. An isolated, individual graphene sheetis commonly referred to as single-layer graphene. A stack of multiplegraphene planes bonded through van der Waals forces in the thicknessdirection is commonly referred to as a multi-layer graphene, typicallyhaving up to 300 layers or graphene planes (<100 nm in thickness), butmore typically up to 30 graphene planes (<10 nm in thickness), even moretypically up to 20 graphene planes (<7 nm in thickness), and mosttypically up to 10 graphene planes (commonly referred to as few-layergraphene in scientific community). Single-layer graphene and multi-layergraphene sheets are collectively called “nano graphene platelets”(NGPs). Graphene or NGP is a new class of carbon nano material (a 2-Dnano carbon) that is distinct from the 0-D fullerene, the 1-D CNT, andthe 3-D graphite.

Our research group pioneered the development of graphene materials andrelated production processes as early as 2002: (1) B. Z. Jang and W. C.Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4,2006), application submitted in October 2012; (2) B. Z. Jang, et al.“Process for Producing Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A.Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets andNanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25,2006).

NGPs are typically obtained by intercalating natural graphite particleswith a strong acid and/or oxidizing agent to obtain a graphiteintercalation compound (GIC) or graphite oxide (GO), as illustrated inFIG. 1(a) (process flow chart) and FIG. 1(b) (schematic drawing). Thisis most often accomplished by immersing natural graphite powder (20 inFIG. 1(a) and 100 in FIG. 1(b)) in a mixture of sulfuric acid, nitricacid (an oxidizing agent), and another oxidizing agent (e.g. potassiumpermanganate or sodium chlorate). The resulting GIC (22 or 102) isactually some type of graphite oxide (GO) particles. This GIC is thenrepeatedly washed and rinsed in water to remove excess acids, resultingin a graphite oxide suspension or dispersion, which contains discreteand visually discernible graphite oxide particles dispersed in water.There are two processing routes to follow after this rinsing step:

Route 1 involves removing water from the suspension to obtain“expandable graphite,” which is essentially a mass of dried GIC or driedgraphite oxide particles. Upon exposure of expandable graphite to atemperature in the range of typically 800-1,050° C. for approximately 30seconds to 2 minutes, the GIC undergoes a rapid expansion by a factor of30-300 to form “graphite worms” (24 or 104), which are each a collectionof exfoliated, but largely un-separated or still interconnected graphiteflakes. A SEM image of graphite worms is presented in FIG. 2(a).

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (26 or 106) that typically havea thickness in the range of 0.125 mm (125 μm)-0.5 mm (500 μm). One maychoose to use a low-intensity air mill or shearing machine to simplybreak up the graphite worms for the purpose of producing the so-called“expanded graphite flakes” (108) which contain mostly graphite flakes orplatelets thicker than 100 nm (hence, not a nano material bydefinition).

Exfoliated graphite worms, expanded graphite flakes, and therecompressed mass of graphite worms (commonly referred to as flexiblegraphite sheet or flexible graphite foil) are all 3-D graphiticmaterials that are fundamentally different and patently distinct fromeither the 1-D nano carbon material (CNT) or the 2-D nano carbonmaterial (graphene).

As disclosed by M. Smalc, et al, U.S. Pat. No. 7,292,441 (Nov. 6, 2007)and U.S. Pat. No. 6,982,874 (Jun. 3, 2006), and J. W. Tzeng, U.S. Pat.No. 6,482,520 (Nov. 19, 2002), these flexible graphite (FG) foils can beused as a heat spreader material, but exhibiting a maximum in-planethermal conductivity of typically less than 500 W/mK (more typically<300 W/mK) and in-plane electrical conductivity no greater than 1,500S/cm. These low conductivity values are a direct result of the manydefects, wrinkled or folded graphite flakes, interruptions or gapsbetween graphite flakes, and non-parallel flakes (e.g. SEM image in FIG.2(b)). Many flakes are inclined with respect to one another at a verylarge angle (e.g. mis-orientation of 20-40 degrees).

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,33 or 112), as disclosed in our U.S. application Ser. No. 10/858,814.Single-layer graphene can be as thin as 0.34 nm, while multi-layergraphene can have a thickness up to 100 nm. In the present application,the thickness of multi-layer NGPs is typically less than 20 nm.

Route 2 entails ultrasonicating the graphite oxide suspension for thepurpose of separating/isolating individual graphene oxide sheets fromgraphite oxide particles. This is based on the notion that theinter-graphene plane separation has been increased from 0.335 nm innatural graphite to 0.6-1.1 nm in highly oxidized graphite oxide,significantly weakening the van der Waals forces that hold neighboringplanes together. Ultrasonic power can be sufficient to further separategraphene plane sheets to form separated, isolated, or discrete grapheneoxide (GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.01%-10% by weight, more typically 0.01%-5%by weight.

For the purpose of defining the claims of the instant application, NGPsinclude single-layer and multi-layer graphene or reduced graphene oxidewith an oxygen content of 0-10% by weight, more typically 0-5% byweight, and preferably 0-2% weight. Pristine graphene has essentially 0%oxygen. Graphene oxide (including RGO) can have 0.01%-46% by weight ofoxygen. The graphene oxide gel, to be described in detail later,typically contains 20-46% by weight oxygen. The graphene oxidegel-derived unitary graphene layer or graphene single crystal of thepresent invention typically has an oxygen content of 0.01% to 5% byweight, more typically less than 2% by weight.

It may be noted that flexible graphite foils (obtained by re-compressingexfoliated graphite worms) for electronic device thermal managementapplications (e.g. as a heat spreader) have the following majordeficiencies:

-   -   (1) As indicated earlier, flexible graphite (FG) foils exhibit a        relatively low thermal conductivity, typically <500 W/mK and        more typically <300 W/mK.    -   (2) Flexible graphite foils are also of low strength and poor        structural integrity. The high tendency for flexible graphite        foils to get torn apart makes them difficult to handle in the        process of integrating them in a microelectronic device.    -   (3) Another very subtle, largely ignored or overlooked, but        critically important feature of FG foils is their high tendency        to get flaky with graphite flakes easily coming off from FG        sheet surfaces and emitting out to other parts of a        microelectronic device. These highly electrically conducting        flakes (typically 1-500 μm in lateral dimensions and >100 nm in        thickness) can cause internal shorting and failure of electronic        devices.    -   (4) For this reason, it is necessary to apply a protective resin        coating onto a surface or on both surfaces of a flexible        graphite foil in order to prevent graphite flakes from being        released. This resin coating is typically not a thermally or        electrically conductive material that is often an undesirable        feature in a situation where high conductivity is required. In        other situations where electrical insulation or isolation is        required, this resin layer can present some issues (e.g.        mis-match in coefficients of thermal expansion and elastic        constants between the FG layer and the resin coating, resulting        in delamination or peeling-off after some number of thermal        cycles).

Other sheet-like graphitic materials that can be used as a heat spreaderor thermal interface material include carbon nano-tube (CNT) paper (e.g.Bucky paper), carbon fiber mat (e.g. carbon nano-fiber or CNF mat), andcarbon paper (e.g. made of short carbon fibers). These graphitic sheetsalso suffer from similar shortcomings as FG foils. For instance,although individual CNTs or CNFs alone can exhibit a high thermalconductivity (1,500-3000 W/mK), the resulting CNT or CNF paper or mattypically exhibit an in-plane thermal conductivity less than 100 W/mKand often less than 10 W/mK, likely due to the few and poor contactsbetween individual CNT or CNF filaments, providing insufficientcross-sections for electron flow or even impeding electron flow.Further, the contact between a sheet-like graphitic layer and a heatsource is usually poor due to limited contact surfaces between such agraphitic layer (e.g. CNT paper) and a rigid device component (e.g. aCPU in a mobile phone). This results in an ineffective heat transferbetween the heat source and the graphitic layer.

Similarly, the NGPs (including discrete platelets of pristine grapheneand GRO), when packed into a film or paper sheet (34 or 114) ofnon-woven aggregates, typically do not exhibit a high thermalconductivity. The thermal conductivity is found to be higher than 1,000W/mK only when the film or paper is cast and pressed into a sheet havinga thickness lower than 10 μm, and higher than 1,500 W/mK only when thefilm or paper is cast and greatly pressed into a sheet having athickness lower than 1 μm. This is reported in our earlier U.S. patentapplication Ser. No. 11/784,606 (Apr. 9, 2007). However, ultra-thin filmor paper sheets (<10 μm) are difficult to produce in mass quantities,and difficult to handle when one tries to incorporate these thin filmsas a heat spreader material during the manufacturing of microelectronicdevices. Further, thickness dependence of thermal conductivity (notbeing able to achieve a high thermal conductivity at a wide range offilm thicknesses) is not a desirable feature.

In general, paper made from platelets of graphene, graphene oxide, andRGO (e.g. those paper sheets prepared by vacuum-assisted filtrationprocess) exhibit many defects, wrinkled or folded graphene sheets,interruptions or gaps between platelets, and non-parallel platelets(e.g. SEM image in FIG. 3(b)), leading to poor thermal conductivity andelectric conductivity. These papers or aggregates of discrete NGP, GO orRGO platelets also have a tendency to get flaky, emitting conductiveparticles into air.

Our earlier application (U.S. application Ser. No. 11/784,606) furtherdisclosed a mat, film, or paper of NGPs infiltrated with a metal, glass,ceramic, resin, and CVD graphite matrix material. Later on, Haddon, etal (US Pub. No. 2010/0140792, Jun. 10, 2010) also reported NGP thin filmand NGP-polymer composites for thermal management applications. Theprocesses used by Haddon et al to produce NGPs are identical to thosedisclosed much earlier by us (Jang, et al. U.S. patent application Ser.No. 10/858,814 (Jun. 3, 2004)). The NGP-polymer composites, as anintended thermal interface material, have very low thermal conductivity,typically <<2 W/mK. The NGP films of Haddon, et al are essentiallynon-woven aggregates of discrete graphene platelets, identical to thoseof our earlier invention (U.S. application Ser. No. 11/784,606). Again,these aggregates have a great tendency to have graphite particlesflaking and separated from the film surface, creating internal shortingproblem for the electronic device containing these aggregates. They alsoexhibit low thermal conductivity unless made into thin films (10 nm-300nm, as reported by Haddon, et al) which are very difficult to handle ina real device manufacturing environment. Balandin, et al (US Pub. No.2010/0085713, Apr. 8, 2010) also disclosed a graphene layer produced byCVD deposition or diamond conversion for heat spreader application. Morerecently, Kim, et al (N. P. Kim and J. P. Huang, “Graphene NanoplateletMetal Matrix,” US Pub. No. 2011/0108978, May 10, 2011) reported metalmatrix infiltrated NGPs. However, metal matrix material is too heavy andthe resulting metal matrix composite does not exhibit a high thermalconductivity.

Another prior art material for thermal management application is thepyrolitic graphite film. The lower portion of FIG. 1(a) illustrates atypical process for producing prior art pyrolitic graphitic films orsheets from a polymer. The process begins with carbonizing a polymerfilm 46 at a carbonization temperature of 500-1,000° C. for 2-10 hoursto obtain a carbonized material 48, which is followed by agraphitization treatment at 2,500-3,200° C. for 5-24 hours to form agraphitic film 50. This is a slow, tedious, and energy-intensiveprocess. Furthermore, carbonization of certain polymers (e.g.polyacrylonitrile) involves the emission of toxic species.

Another type of pyrolytic graphite is produced by high temperaturedecomposition of hydrocarbon gases in vacuum followed by deposition ofthe carbon atoms to a substrate surface. This is essentially a chemicalvapor deposition (CVD) process. In particular, highly oriented pyroliticgraphite (HOPG) is the material produced by the application of uniaxialpressure on deposited pyrocarbon or pyrolytic graphite at very hightemperatures (typically 3,000-3,300° C.). This entails athermo-mechanical treatment of combined mechanical compression andultra-high temperature for an extended period of time in a protectiveatmosphere; a very expensive, energy-intensive, and technicallychallenging process. The process requires high vacuum and ultra-hightemperature equipment that is not only very expensive to make but alsovery expensive and difficult to maintain. Even with such extremeprocessing conditions, the resulting PG (including HOPG) still possessesmany defects, grain boundaries, and mis-orientations (neighboringgraphene planes not parallel to each other), resulting inless-than-satisfactory in-plane properties. Typically, the best preparedHOPG sheet or block remains far from being a graphite single crystal;instead, it typically still contains many grains or single crystals anda vast amount of grain boundaries and defects. In general, the PG orHOPG is free from any element than carbon.

Similarly, the most recently reported graphene thin film (<2 nm)prepared by catalytic CVD of hydrocarbon gas (e.g. C₂H₄) on Ni or Cusurface is not a single-grain crystal, but a poly-crystalline structurewith many grain boundaries and defects [e.g., Piran R. Kidambi, et al.,“The Parameter Space of Graphene Chemical Vapor Deposition onPolycrystalline Cu,” The Journal of Physical Chemistry C2012 116 (42),22492-22501]. With Ni or Cu being the catalyst, carbon atoms obtainedvia decomposition of hydrocarbon gas molecules at 800-1,000° C. aredeposited onto Ni or Cu foil surface to form a sheet of single-layer orfew-layer graphene that is poly-crystalline. The grains are typicallymuch smaller than 100 μm in size and, more typically, smaller than 10 μmin size. These graphene thin films, being optically transparent andelectrically conducting, are intended for touch screen (to replaceindium-tin oxide or ITO glass) or semiconductor (to replace silicon, Si)applications. However, these polycrystalline graphene films are notsufficiently thermally conducting (too many grains or too much grainboundaries, and all grains being oriented in different directions) andnot sufficiently thick for use as a heat spreader in an electronicdevice.

Thus, it is an object of the present invention to provide a grapheneoxide (GO) gel-derived unitary or monolithic film, which exhibits athermal conductivity comparable to or greater than that of the PG, HOPG,or CVD graphene film.

It is a specific object of the present invention to provide a GOgel-derived unitary or monolithic entity, which has the followingcharacteristics (separately or in combination): (1) This unitary entityis an integrated graphene object that is either a graphene singlecrystal (single grain only) or a poly-crystal (multiple grains buthaving incomplete grain boundaries) with all graphene planes in allgrains being essentially oriented parallel to one another (thecrystallographic c-axis of all grains being essentially parallel to oneanother). (2) This integrated graphene entity is not an aggregate orstack of multiple discrete graphite flakes or discrete platelets ofgraphene or GO, and does not contain any discernible or discreteflake/platelet. (3) This integrated graphene entity is not made bygluing or bonding discrete flakes/platelets together with a binder,linker, or adhesive. Instead, GO molecules in the GO gel are merged,mainly edge-to-edge through joining or forming of covalent bonds withone another, into an integrated graphene entity, without using anyexternally added linker or binder molecules or polymers. (4) Thisunitary or monolithic graphene entity (a single crystal or poly-crystalwith all graphene planes having crystallographic c-axis essentiallyparallel to each other) is derived from a GO gel, which is in turnobtained from heavy oxidation of natural graphite or artificial graphiteparticles originally having multiple graphite crystallites. Prior tobeing chemically oxidized to become GO gel, these starting graphitecrystallites have an initial length (L_(a) in the crystallographica-axis direction), initial width (L_(b) in the b-axis direction), andthickness (L_(c) in the c-axis direction). This unitary graphene entitytypically has a length or width significantly greater than the L_(a) andL_(b) of the original crystallites.

The present invention also provides a method or process for producingsuch a GO gel-derived unitary or monolithic graphene entity, or agraphene single crystal (including a graphene poly-crystal with anincomplete grain boundary). The process begins with preparation of amass of GO gel preferably in a layer form (preferably less than 10 mm inthickness, more preferably less than 1 mm, and most preferably less than500 μm in thickness prior to drying). The liquid component of this GOgel is then partially or totally removed and, concurrently orsequentially, this GO layer is thermally converted to an integratedgraphene film obtained by heat-treating graphene oxide gel to chemicallymerge individual graphene oxide molecules primarily in an edge-to-edgemanner.

Another object of the present invention is to provide a cost-effectiveprocess of producing GO-derived graphene monolith that exhibits acombination of exceptional thermal conductivity, electricalconductivity, mechanical strength, surface hardness, and scratchresistance unmatched by any thin-film graphitic material of comparablethickness range.

In particular, the present invention provides a process for producing aunitary or monolithic graphene layer or graphene single crystal from aGO gel. This process does not involve or require an ultrahightemperature as is absolutely required of the processes for producingpyrolytic graphite (including HOPG) from either carbonized polymers(e.g. polyimide) or using the CVD deposition. The presently inventedprocess is simple, less energy-intensive, and highly scalable.

This thermally and electrically conductive graphene monolith can be usedfor thermal management applications (e.g. for use as a heat spreader) ina microelectronic device, such as a mobile phone (including a smartphone), a notebook computer, a tablet, an e-book, a telecommunicationdevice, and any hand-held computing device or portable microelectronicdevice.

It is another object of the present invention to provide a GO-derivedunitary graphene entity that exhibits a combination of exceptionalthermal conductivity, electrical conductivity, mechanical strength,surface smoothness, surface hardness, and scratch resistance unmatchedby any thin-film material of comparable thickness range.

It is a specific object of the present invention to provide a highlyconductive graphene monolith thin-film sheet that meets the followingtechnical requirements (a) in-plane thermal conductivity greater than600 W/mK (preferably greater than 1,000 W/mK, and further preferablygreater than 1,700 W/mK); (b) in-plane electrical conductivity greaterthan 2,000 S/cm (preferably >3,000 S/cm, more preferably >5,000 S/cm,and most desirably >10,000 S/cm); (c) Rockwell surface hardnessvalue >60 (preferably >80); and/or (d) a tensile strength greater than10 MPa (preferably >40 MPa, more preferably >60 MPa, and mostpreferably >100 MPa).

SUMMARY OF THE INVENTION

The present invention provides a unitary graphene layer or graphenesingle crystal containing closely packed and bonded parallel grapheneplanes having an inter-graphene plane spacing of 0.335 to 0.40 nm and anoxygen content of 0.01% to 10% by weight. This unitary graphene layer orgraphene single crystal is obtained from heat-treating a graphene oxidegel at a temperature higher than 100° C., wherein an averagemis-orientation angle between two graphene planes is less than 10degrees, preferably and typically less than 5 degrees. The thickness ofthis unitary graphene entity or graphene single crystal is typicallygreater than 1 nm, and more typically greater than 10 nm (opticallyopaque), and further more typically greater than 10 μm for thermalmanagement applications.

The graphene single crystal herein refers to the single-grain orsingle-domain graphene or poly-crystalline structure (but having anincomplete grain boundary) in which all the graphene planes in allgrain(s) are essentially parallel to one another.

The graphene oxide gel-derived unitary or monolithic graphene layer orgraphene single crystal has a unique combination of outstanding thermalconductivity, electrical conductivity, mechanical strength, scratchresistance, and elimination of the possibility of having surfacegraphite flakes or particles to “flake off” (actually, there is nodiscrete flake/platelet to be peeled therefrom).

The graphene oxide (GO) gel-derived unitary or monolithic entity has thefollowing characteristics (separately or in combination):

-   -   (1) This unitary entity is an integrated graphene object that is        either a graphene single crystal or a poly-crystal having        multiple grains (but with incomplete or poorly delineated grain        boundaries). This unitary graphene entity is composed of        multiple graphene planes that are essentially oriented parallel        to one another. Specifically, the crystallographic c-axis        directions of all the graphene planes in all grains are        essentially parallel to one another.    -   (2) In contrast to the paper-like sheets of expanded graphite        flakes or graphene platelets (e.g. those prepared by a        paper-making process), this integrated graphene entity is not an        aggregate or stack of multiple discrete graphite flakes or        discrete platelets of graphene, GO, or RGO. This is a single        graphene entity or monolith, not a simple aggregate of multiple        graphite flakes (FG foil) or graphene sheets (graphene paper).        This unitary graphene entity does not contain discrete graphite        flakes or discrete nano graphene platelets (platelets of        pristine graphene, graphene oxide, and reduced graphene oxide)        dispersed therein.    -   (3) In other words, this graphene monolith is not the result of        exfoliating the graphene sheets or graphite flakes (that        constitute the original structure of graphite particles) and        then simply re-orienting these discrete sheets/flakes along one        direction. Such an aggregating procedure leads to a simple        collection or stack of discrete flakes/sheets/platelets that can        be detected or discerned with an un-assisted eye or under a        low-magnification optical microscope (×100-×1000).        -   Contrarily, the original graphite particles are heavily            oxidized, to the extent that practically every one of the            original graphene planes has been oxidized and isolated from            one another to become individual molecules that possess            highly reactive functional groups at the edge and, mostly,            on graphene planes as well. These individual hydrocarbon            molecules (containing elements such as O and H, not just            carbon atoms) are dissolved in the reaction medium (e.g.            mixture of water and acids) to form a gel-like mass, herein            referred to as GO gel. This gel is then cast onto a smooth            substrate surface, with the liquid components removed to            form a dried GO layer. When properly dispersed and heated on            a solid substrate surface, these highly reactive molecules            react and join with one another mostly in lateral directions            along graphene planes (in an edge-to-edge manner) and, in            some cases, between graphene planes as well. These linking            and merging reactions proceed in such a manner that the            molecules are chemically merged, linked, and integrated into            one single entity or monolith (not just physically stacked            or packed together). The molecules completely lose their own            original identity and they no longer are discrete            sheets/platelets/flakes. There is only one single layer-like            structure (unitary graphene entity) that is essentially one            huge molecule or just a few giant molecules with an            essentially infinite molecular weight. This may also be            described as a graphene single crystal (with only one grain            in the entire structure or entity, or a poly-crystal having            several grains, but typically no discernible, well-defined            grain boundaries, e.g. FIG. 3(f)). All the constituent            graphene planes are very large in lateral dimensions (length            and width) and are essentially parallel to one another.        -   In-depth X-ray diffraction, atomic force microscopy, and            electron microscopy (including selected area diffraction)            studies indicate that the graphene monolith is composed of            several huge graphene planes (with length/width            typically >>100 μm, more typically >>1 mm, and most            typically >>1 cm). These giant graphene planes are stacked            and bonded along the thickness direction (crystallographic            c-axis direction) through not just the van der Waals forces            in conventional graphite crystallites, but also covalent            bonds, Not to be limited by theory, but the studies based on            combined Raman, FTIR, and electron spectroscopy for chemical            analysis (ESCA) appear to indicate the co-existence of sp²            (dominating) and sp³ (weak but existing) electronic            configurations, not just the conventional sp² alone in            graphite.    -   (4) This integrated graphene entity is not made by gluing or        bonding discrete flakes/platelets together with a binder,        linker, or adhesive. Instead, GO molecules in the GO gel are        merged, mainly edge-to-edge through joining or forming of        covalent bonds with one another, into an integrated graphene        entity, without using any externally added linker or binder        molecules or polymers.    -   (5) This unitary or monolithic graphene entity is a single        crystal or poly-crystal (having poorly defined or incomplete        grain boundaries) with the crystallographic c-axis in all grains        being essentially parallel to each other. This entity is derived        from a GO gel, which is in turn obtained from natural graphite        or artificial graphite particles originally having multiple        graphite crystallites. Prior to being chemically oxidized, these        starting graphite crystallites have an initial length (L_(a) in        the crystallographic a-axis direction), initial width (L_(b) in        the b-axis direction), and thickness (L_(c) in the c-axis        direction). The resulting unitary graphene entity typically has        a length or width significantly greater than the L_(a) and L_(b)        of the original crystallites. The length/width of this unitary        graphene entity or that of a graphene single crystal is        typically greater than the L_(a) and L_(b) of the original        crystallites. Even the individual grains in a poly-crystalline        unitary graphene entity have a length or width significantly        greater than the L_(a) and L_(b) of the original crystallites.        They can be as large as the length or width of the unitary        graphene entity itself, not just 2 or 3 times higher than the        initial L_(a) and L_(b) of the original crystallites.

The graphene oxide-derived monolithic graphene layer preferably has athickness less than 200 μm for a heat spreader application, but it canbe thicker. Further preferably, the monolithic graphene layer orgraphene single crystal has a thickness greater than 1 μm, but less than200 μm. In some applications, the thickness is preferably greater than10 μm. The thickness range of 20-100 μm is particularly useful formobile device thermal management applications.

The unitary graphene sheet of the present invention has overcome all themajor problems associated with the flexible graphite foil produced byre-compression of exfoliated graphite worms or exfoliated graphiteflakes of natural graphite and/or artificial graphite. The flexiblegraphite sheet or foil prepared by re-compressing (e.g. roll-pressing)exfoliated graphite worms or flakes has a great tendency to flake off,emitting graphite flakes into air and eventually relocating to adangerous spot (e.g. where the presence of graphite flakes could causeinternal short-circuiting). Further, flexible graphite sheets or foilsare relatively brittle and weak, and hence are difficult to handle in anactual microelectronic device manufacturing environment. They also donot possess high thermal conductivity (most typically <300 W/mK). Theseand other major issues associated with the use of flexible graphitesheets in a microelectronic device for a thermal management purpose havebeen effectively overcome surprisingly by the presently invented unitarygraphene body.

The unitary graphene sheet is derived from a graphene oxide gel, whichis produced from particles of natural graphite or artificial graphitecomposed of multiple graphite crystallites. These crystallites typicallyhave an initial length L_(a) (in the crystallographic a-axis direction)of less than 100 μm (more typically less than 10 μm), an initial widthL_(b) in the b-axis direction also of more typically less than 10 μm,and a thickness L_(c) in the c-axis direction (typically 0.2 to 10 μm).However, the presently invented GO-derived unitary graphene layer orgraphene single crystal typically has a length or width at least greaterthan twice (more typically significantly greater than 3 times) theinitial L_(a) or twice (more typically >3 times) the L_(b) of thegraphite crystallites of the starting materials. The unitary graphenelayer or graphene single crystal typically has a length or width no lessthan 10 μm, more typically no less than 100 μm, and even more typicallyno less than 1 cm. They often are extended to cover the entire width ofthe original GO gel layer deposited on a substrate surface, which canbe >100 cm as desired.

As a preferred processing condition for the unitary graphene layer orgraphene single crystal, the heat-treating temperature for GO is from100° C. to 1,000° C. and the unitary graphene layer or graphene singlecrystal has a thermal conductivity greater than 600 W/mK or electricalconductivity greater than 2,000 S/cm. Alternatively, the heat-treatingtemperature is from 1,000° C. to 1,500° C. and the resulting unitarygraphene layer or graphene single crystal typically has a thermalconductivity greater than 1,300 W/mK or electrical conductivity greaterthan 3,000 S/cm. With a heat-treating temperature of from 1500° C. to2,500° C., the unitary graphene layer or graphene single crystal has athermal conductivity greater than 1,600 W/mK or electrical conductivitygreater than 5,000 S/cm (or even >8,000 S/cm). With a heat-treatingtemperature of from 2,500° C. to 3,250° C., the unitary graphene layeror graphene single crystal has a thermal conductivity greater than 1,700W/mK or electrical conductivity greater than 8,000 S/cm (typicallygreater than 10,000 S/cm and, in many cases, greater than 15,000 S/cm).

The unitary graphene layer or graphene single crystal can have athickness as low as 1 nm, but preferably >10 nm, more preferably >1 μm,even more preferably >10 μm. For use as a heat spreader, the thicknessis typically in the range of 10-200 μm, but most typically or desirablybetween 20 and 100 μm. As indicated earlier, the unitary graphene layeror graphene single crystal typically has a lateral dimension (length orwidth) significantly greater than 100 μm.

The unitary graphene layer or graphene single crystal typically has anoxygen content from 0.01% to 5% by weight, more typically from 0.01% to2% by weight. If the re-graphitization temperature exceeds 2,000° C. andis conducted under very strict protective atmosphere or extremely highvacuum conditions, one can essentially eliminate oxygen.

For the preparation of the unitary graphene layer or graphene singlecrystal, the graphene oxide gel is composed of graphene oxide moleculesdispersed in an acidic medium having a pH value of no higher than 5 andthe graphene oxide molecules have an oxygen content no less than 20% byweight while in a gel state.

The GO gel is obtained by immersing a graphitic material in a powder orfibrous form (e.g. natural or artificial graphite powder or graphitefibers) in an oxidizing liquid medium in a reaction vessel at a reactiontemperature for a length of time sufficient to obtain a graphene oxidegel composed of graphene oxide molecules dispersed in the liquid medium.The graphene oxide molecules preferably and typically have an oxygencontent no less than 20% by weight (typically 20%-46% by weight ofoxygen) and a molecular weight less than 43,000 g/mole while in a gelstate. Preferably, graphene oxide molecules have a molecular weight lessthan 4,000 g/mole while in a gel state, more preferably between 200g/mole and 4,000 g/mole while in a gel state.

The unitary graphene layer or graphene single crystal is produced bydepositing a layer of graphene oxide gel onto a surface of a substrateand removing the residual liquid from this layer of deposited grapheneoxide gel. This is followed by subjecting this graphene-oxide layer to aheat treatment temperature of at least 100-150° C. for thermal reductionand/or re-graphitization. A good heat treatment temperature is from 300°C. to 1,500° C. for re-graphitization. Although not required, the heattreatment temperature may be higher than 1,500° C. forre-graphitization, or may be in the range of from 1,500° C. to 2,500° C.A temperature higher than 2,500° C. may be used if so desired.

The starting materials for the preparation of graphene oxide gel includea graphitic material selected from natural graphite, artificialgraphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead,soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbonnano-tube, or a combination thereof.

The unitary graphene layer or graphene single crystal with a thicknessgreater than 200 μm shows a surprisingly high Rockwell hardness value,typically greater than 60 and often greater than 100. This isunprecedented since prior art flexible graphite foil, pyrolyticgraphite, or bulk graphite does not show such a high hardness.

The unitary graphene layer or graphene single crystal of the presentinvention can exhibit an electrical conductivity greater than 1,500S/cm, a thermal conductivity greater than 600 W/mK, a physical densitygreater than 1.8 g/cm³, and/or a tensile strength greater than 40 MPa.

With a higher re-graphitization temperature, the graphene monolithic canhave an electrical conductivity greater than 3,000 S/cm, a thermalconductivity greater than 1,000 W/mK, a physical density greater than2.0 g/cm³, and/or a tensile strength greater than 80 MPa. It can evenexhibit an electrical conductivity greater than 5,000 S/cm, a thermalconductivity greater than 1,500 W/mK, a physical density greater than2.1 g/cm³, and/or a tensile strength greater than 100 MPa.

The present invention also provides a process for producing theaforementioned unitary graphene layer or graphene single crystal. Theprocess comprises: (a) preparing a graphene oxide gel having grapheneoxide molecules dispersed in a fluid medium, wherein the graphene oxidegel is optically transparent or translucent; (b) depositing a layer ofthe graphene oxide gel onto a surface of a supporting substrate to forma deposited graphene oxide gel thereon; (c) partially or completelyremoving the fluid medium from the deposited graphene oxide gel layer toform a graphene oxide layer; and (d) heat-treating the graphene oxidelayer to form the unitary graphene layer or graphene single crystal. Theprocess further comprises a step of compressing the graphene oxide layer(e.g. via roll-pressing through a set or multiple sets of rollers).

In particular, the graphene oxide gel is prepared by immersing agraphitic material in a powder or fibrous form in an oxidizing liquid toform an initially optically opaque suspension in a reaction vessel at areaction temperature for a length of time sufficient to obtain agraphene oxide gel that is optically transparent or translucent. Thegraphene oxide gel is composed of graphene oxide molecules dispersed inan acidic medium having a pH value of no higher than 5 and the grapheneoxide molecules have an oxygen content no less than 20% by weight(typically from 20% to approximately 46% by weight.

Typically, the graphene oxide gel is prepared by immersing a graphiticmaterial in an oxidizing agent to form an initially optically opaquesuspension and allowing an oxidizing reaction to proceed until anoptically transparent or translucent solution is formed. The graphiticmaterial is selected from natural graphite, artificial graphite,meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, softcarbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbonnano-tube, or a combination thereof.

The process is preferably a roll-to-roll process, wherein steps (b) and(c) include feeding a sheet of a solid substrate material from a rollerto a deposition zone, depositing a layer of graphene oxide gel onto asurface of the sheet of solid substrate material to form a grapheneoxide gel layer thereon, drying the graphene oxide gel to form a driedgraphene oxide layer deposited on the substrate surface, and collectinggraphene oxide layer-deposited substrate sheet on a collector roller.The process preferably includes an additional step of compressing thegraphene oxide layer prior to being collected on the collector roller.

This graphene oxide gel has the characteristics that it is opticallytransparent or translucent and visually homogeneous with no discerniblediscrete graphene or graphene oxide sheets dispersed therein. Incontrast, conventional suspension of discrete graphene or graphene oxidesheets, or graphite flakes looks opaque, dark, black or heavy brown incolor with individual graphene sheets, graphene oxide sheets, orgraphite flakes being discernible or recognizable with naked eyes.

The graphene oxide molecules dissolved in the liquid medium of agraphene oxide gel are aromatic chains that have an average number ofbenzene rings in the chain typically less than 1000, more typically lessthan 500, and most typically less than 100. Most of the molecules havemore than 5 or 6 benzene rings (mostly >10 benzene rings) from combinedatomic force microscopy, high-resolution TEM, and molecular weightmeasurements. These benzene-ring type of aromatic molecules have beenheavily oxidized and contain functional groups, such as —COOH and —OHand, therefore, are “soluble” (not just dispersible) in polar solvents,such as water.

These soluble molecules behave like resins and are surprisingly capableof forming a coherent layer of graphene oxide of good structuralintegrity and high thermal conductivity. By contrast, conventionaldiscrete graphene or graphene oxide sheets and graphite flakes do nothave any self-adhesion or cohesion power. These sheets or flakes wouldjust form a loosely packed mass of un-bonded particles that does nothave any structural integrity.

The present invention also provides a heat spreader or heat sink productfor use in a hand-held device, such as a power tool, a microelectronicor telecommunication device (e.g. mobile phone, tablet, laptop computer,LCD display, etc), a light-emitting diode (LED) lighting device orsystem. The light weight (lower density compared to metal and ceramicmaterials), exceptional thermal conductivity, relatively high structuralintegrity, superior surface hardness and scratch resistance, andeliminated or significantly reduced tendency to emit free graphite orcarbon particles into air make the invented graphene oxide-coatedgraphitic layer an ideal thermal management material.

In summary, the present invention provides a unitary graphene layer forheat-spreading applications. In one preferred embodiment, this graphenemonolith contains closely packed, gap-free, and chemically bondedparallel graphene planes that have an inter-graphene plane spacing of0.335 to 0.50 nm (more typically 0.336 to 0.50 nm) and an oxygen contentless than 1% by weight. The unitary graphene layer typically has athickness greater than 10 nm, contains no discrete graphite flake orgraphene platelet dispersed therein, and has an average mis-orientationangle between two graphene planes less than 10 degrees. This graphenemonolith is obtained from heat-treating a graphene oxide gel at atemperature higher than 500° C.

Another preferred embodiment of the present invention is a unitarygraphene layer (particularly useful for heat-spreading applications)that contains closely packed and chemically bonded parallel grapheneplanes having an inter-graphene plane spacing of 0.335 to 0.50 nm and anoxygen content less than 1% by weight (typically from 0.001% to 1%).This unitary graphene layer or monolith contains a poly-crystal orpoly-grain structure that has an incomplete grain boundary, contains nodiscrete graphite flake or graphene platelet dispersed therein, and isobtained from heat-treating a graphene oxide gel at a temperature higherthan 500° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (a) A flow chart illustrating various prior art processes ofproducing exfoliated graphite products (flexible graphite foils andflexible graphite composites) and pyrolytic graphite (bottom portion),along with processes for producing graphene oxide gel and GO gel-coatedlaminates; (b) Schematic drawing illustrating the processes forproducing graphite or graphene oxide paper, mat, film, and membrane ofsimply aggregated flakes/platelets. All processes begin withintercalation and/or oxidation treatment of graphitic materials (e.g.natural graphite particles).

FIG. 2 (a) A SEM image of a graphite worm sample after thermalexfoliation of graphite intercalation compounds (GICs) or graphite oxidepowders; (b) An SEM image of a cross-section of a flexible graphitefoil, showing many graphite flakes with orientations not parallel to theflexible graphite foil surface and also showing many defects, kinked orfolded flakes.

FIG. 3 (a) A SEM image of a GO-derived graphene monolithic whereinmultiple graphene sheets, originally 30 nm-2 μm in lateral dimension,have been oxidized, exfoliated, re-oriented, and seamlessly merged intocontinuous-length graphene sheets or layers that can run for hundreds ofcentimeters wide or long (only a 120 μm or 0.12 mm width of a 25-cm wideunitary graphene layer being shown in this SEM image); (b) A SEM imageof a cross-section of a graphene paper/film prepared from discretegraphene sheets/platelets using a paper-making process (e.g.vacuum-assisted filtration). The image shows many discrete graphenesheets being folded or interrupted (not integrated), with orientationsnot parallel to the film/paper surface and having many defects orimperfections; (c) Schematic drawing and an attendant SEM image toillustrate the formation process of a unitary graphene entity orgraphene single crystal that is composed of multiple graphene planesthat are parallel to one another and are chemically bonded in thethickness-direction or crystallographic c-axis direction; (d) Schematicof the prior art graphene poly-crystal obtained by CVD of hydrocarbon ona catalytic surface (e.g. Cu or Ni); (e) Schematic of a graphene singlecrystal of the present invention; (f) Schematic of another graphenesingle crystal of the present invention (a “poly-crystal” withincomplete grain boundaries).

FIG. 4 (a) Thermal conductivity values of the GO-derived single unitarygraphene layer (▴), GO paper (▪), and FG foil (♦) plotted as a functionof the final heat treatment temperature for graphitization orre-graphitization; (b) Thermal conductivity values of the GO-derivedunitary graphene layer (▪) and the polyimide-derived pyrolytic graphite(PG) heat-treated for one hour (x) and for 3 hours (▴), all plotted as afunction of the final graphitization or re-graphitization temperature;(c) Electric conductivity values of the GO-derived unitary graphenelayer (♦), GO paper (▪), and FG foil (x) plotted as a function of thefinal graphitization or re-graphitization temperature. Note: symboldesignations varied from (a) to (c).

FIG. 5 X-ray diffraction curves of (a) a GO film, (b) GO film thermallyreduced at 150° C. (partially re-graphitized), and (c) highly reducedand re-graphitized GO film (a unitary graphene layer).

FIG. 6 (a) Inter-graphene plane spacing measured by X-ray diffraction;(b) the oxygen content in the GO-derived unitary graphene layer; and (c)thermal conductivity of GO-derived unitary graphene layer andcorresponding flexible graphite (FG) foil, all plotted as a function ofthe final heat treatment temperature.

FIG. 7 Surface temperature fields of two identical smart phones runningthe same video programs for 10 minutes. One smart phone (top image)contains 2 sheets of flexible graphite (FG) foils disposed between theCPU and the casing, showing an external surface temperature as high as38.6° C. The other smart phone (bottom image) contains one sheet ofunitary graphene layer-coated FG foil, showing an external surfacetemperature of 25.4° C.

FIG. 8 (a) Thermal conductivity values of the GO layer alone (▪),GO-coated flexible graphite (FG) foil (♦), and FG foil alone (▴) plottedas a function of the final graphitization or re-graphitizationtemperature, along with theoretically predicted values (x) based on arule-of-mixture law (graphitization time=1 hour for all specimens); (b)Thermal conductivity values of the GO layer alone (▪), GO-coatedflexible graphite (FG) foil (♦), and polyimide-derived pyrolyticgraphite (PG) plotted as a function of the final graphitization orre-graphitization temperature for one hour, along with those of PGgraphitized for 3 hours; (c) Electric conductivity values of the GOlayer alone (♦), GO-coated flexible graphite (FG) foil (▴), and FG foilalone (x) plotted as a function of the final graphitization orre-graphitization temperature, along with theoretically predicted values(▪) based on a rule-of-mixture law. Note: symbol designations variedfrom (a) to (c).

FIG. 9 (a) Tensile strength, (b) scratch visibility, (c) scratch depth,and (d) Rockwell hardness of a series of GO-derived unitarygraphene-coated FG foils plotted as a function of the coating-to-corelayer thickness ratio.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a unitary graphene layer (graphenemonolith) or graphene single crystal containing closely packed andchemically bonded graphene planes that run essentially parallel to eachother. These graphene planes cover a wide area with the length and widthcapable of being extended to cover the entire specimen length or width.This graphene monolith has an inter-graphene plane spacing of 0.335 to0.40 nm as determined by X-ray diffraction, and an oxygen content of0.01% to 10% by weight. This unitary graphene layer or graphene singlecrystal is obtained from heat-treating a graphene oxide gel at atemperature higher than 100° C., typically higher than 150° C., and moretypically in the range of 1,000° C. to 1,500° C. Although not necessary,the heat treatment temperature can go above 1,500° C., even above 2,500°C. The average mis-orientation angle between two graphene planes in thisgraphene monolith is less than 10 degrees, preferably and typically lessthan 5 degrees. Most of the graphene layers are essentially parallel toone another with zero mis-orientation angle. In contrast, themis-orientation angles in conventional flexible graphite sheets aresignificantly higher than 10 degrees.

The graphene oxide (GO) gel-derived unitary or monolithic entity has thefollowing characteristics (separately or in combination):

-   -   (1) This unitary graphene entity is an integrated graphene        object that is either a graphene single crystal or a        poly-crystal having multiple grains that are essentially        oriented parallel to one another. The crystallographic c-axis        directions of all grains and all their constituent graphene        planes are essentially parallel to one another. It may be noted        that the grains in a graphene poly-crystal have very poorly        delineated or incomplete grain boundaries. These grains are        essentially a single grain with some residual demarcation lines        (e.g., FIG. 3(f)). Such type of graphene poly-crystal is best        described as a graphene single crystal with some aligned but        sporadic defects. This conclusion was drawn after an extensive        investigation using a combination of SEM, TEM, selected area        diffraction (with a TEM), X-ray diffraction, atomic force        microscopy (AFM), Raman spectroscopy, and FTIR.    -   (2) The paper-like sheets of expanded graphite flakes (flexible        graphite foils) or graphene or GO platelet-based paper (e.g.        those prepared by a paper-making process) are a simple,        un-bonded aggregate/stack of multiple discrete graphite flakes        or discrete platelets of graphene, GO, or RGO. In contrast, this        unitary graphene entity is a fully integrated, single graphene        entity or monolith containing no discrete flakes or platelets.    -   (3) In other words, this graphene monolith is not the result of        exfoliating the graphene sheets or graphite flakes (that        constitute the original structure of graphite particles) and        then simply re-orienting these discrete sheets/flakes along one        direction. The flakes or sheets of the resulting aggregates        (paper, membrane, or mat) remain as discrete        flakes/sheets/platelets even with an un-assisted eye or under a        low-magnification optical microscope (×100-×1000).        -   Contrarily, for the preparation of the presently invented            unitary graphene structure, the original graphite particles            are heavily oxidized, to the extent that practically every            one of the original graphene planes has been oxidized and            isolated from one another to become individual molecules            that possess highly reactive functional groups at the edge            and, mostly, on graphene planes as well. These individual            hydrocarbon molecules (containing elements such as O and H,            in addition to carbon atoms) are dissolved in the reaction            medium (e.g. mixture of water and acids) to form a gel-like            mass, herein referred to as GO gel. This gel is then cast            onto a smooth substrate surface and the liquid components            are then removed to form a dried GO layer. When heated,            these highly reactive molecules react and join with one            another mostly in lateral directions along graphene planes            (in an edge-to-edge manner) and, in some cases, between            graphene planes as well. These linking and merging reactions            proceed in such a manner that the molecules are merged,            linked, and integrated into one single entity or monolith.            The molecules completely lose their own identity and they no            longer are discrete sheets/platelets/flakes. There is only            one single layer-like structure (unitary graphene entity)            that is one huge molecule or just a few giant molecules with            an essentially infinite molecular weight. This may also be            described as a graphene single crystal (with only one grain            in the entire structure or entity, or a poly-crystal (with            several grains, but typically no discernible, well-defined            grain boundaries). All the constituent graphene planes are            very large in lateral dimensions (length and width) and are            essentially parallel to one another.        -   In-depth studies using a combination of SEM, TEM, selected            area diffraction, X-ray diffraction, AFM, Raman            spectroscopy, and FTIR indicate that the graphene monolith            is composed of several huge graphene planes (with            length/width typically >>100 μm, more typically >>1 mm, and            most typically >>1 cm). These giant graphene planes are            stacked and bonded along the thickness direction            (crystallographic c-axis direction) through not just the van            der Waals forces in conventional graphite crystallites, but            also covalent bonds, Not to be limited by theory, but Raman            and FTIR spectroscopy studies appear to indicate the            co-existence of sp² (dominating) and sp³ (weak but existing)            electronic configurations, not just the conventional sp²            alone in graphite.    -   (4) This integrated graphene entity is not made by gluing or        bonding discrete flakes/platelets together with a binder,        linker, or adhesive. Instead, GO molecules in the GO gel are        merged, mainly edge-to-edge through joining or forming of        covalent bonds with one another, into an integrated graphene        entity, without using any externally added linker or binder        molecules or polymers.    -   (5) This unitary or monolithic graphene entity is a single        crystal (e.g. FIG. 3(e)) or poly-crystal (having incomplete        grain boundaries, FIG. 3(f)) with the crystallographic c-axis in        all grains being essentially parallel to each other. This entity        is derived from a GO gel, which is in turn obtained from natural        graphite or artificial graphite particles originally having        multiple graphite crystallites. Prior to being chemically        oxidized, these starting graphite crystallites have an initial        length (L_(a) in the crystallographic a-axis direction), initial        width (L_(b) in the b-axis direction), and thickness (L_(c) in        the c-axis direction). Upon heavy oxidation, these initially        discrete graphite particles are chemically transformed into        highly aromatic graphene oxide molecules having a significant        concentration of edge- or surface-borne functional groups (e.g.        —OH, —COOH, etc.). These aromatic GO molecules in the GO gel        have lost their original identity of being part of a graphite        particle or flake. Upon removal of the liquid component from the        GO gel, the resulting GO molecules form an essentially amorphous        structure. Upon heat treatment (re-graphitization treatment),        these GO molecules are chemically merged and linked into a        unitary or monolithic graphene entity.        -   The resulting unitary graphene entity typically has a length            or width significantly greater than the L_(a) and L_(b) of            the original crystallites. The length/width of this unitary            graphene entity or that of a graphene single crystal is            typically greater than the L_(a) and L_(b) of the original            crystallites. Even the individual grains in a            poly-crystalline unitary graphene entity have a length or            width significantly greater than the L_(a) and L_(b) of the            original crystallites. They can be as large as the length or            width of the unitary graphene entity itself, not just 2 or 3            times higher than the initial L_(a) and L_(b) of the            original crystallites.    -   (6) Due to these unique chemical composition (including oxygen        content), morphology, crystal structure (including        inter-graphene spacing), and structural features (e.g. defects,        incomplete or lack of grain boundaries, chemical bonding and no        gap between graphene sheets, and no interruptions in graphene        planes), the graphene oxide gel-derived unitary or monolithic        graphene layer has a unique combination of outstanding thermal        conductivity, electrical conductivity, mechanical strength, and        scratch resistance (including elimination of the tendency for        surface graphite flakes or particles to “flake off” since there        is essentially no discrete flake or platelet in this graphene        monolith structure).

The aforementioned features are further described and explained indetails as follows:

As illustrated in FIG. 1(b), a graphite particle (e.g. 100) is typicallycomposed of multiple graphite crystallites or grains A graphitecrystallite is made up of layer planes of hexagonal networks of carbonatoms. These layer planes of hexagonally arranged carbon atoms aresubstantially flat and are oriented or ordered so as to be substantiallyparallel and equidistant to one another in a particular crystallite.These layers of carbon atoms, commonly referred to as graphene layers orbasal planes, are weakly bonded together in their thickness direction(crystallographic c-axis direction) by weak van der Waals forces andgroups of these graphene layers are arranged in crystallites.

The graphite crystallite structure is usually characterized in terms oftwo axes or directions: the c-axis direction and the a-axis (or b-axis)direction. The c-axis is the direction perpendicular to the basalplanes. The a- or b-axes are the directions parallel to the basal planes(perpendicular to the c-axis direction).

A highly ordered graphite particle can consist of crystallites of aconsiderable size, having a length of L_(a) along the crystallographica-axis direction, a width of L_(b) along the crystallographic b-axisdirection, and a thickness L_(c) along the crystallographic c-axisdirection. The constituent graphene planes of a crystallite are highlyaligned or oriented with respect to each other and, hence, theseanisotropic structures give rise to many properties that are highlydirectional. For instance, the thermal and electrical conductivity of acrystallite are of great magnitude along the plane directions (a- orb-axis directions), but relatively low in the perpendicular direction(c-axis). As illustrated in the upper-left portion of FIG. 1(b),different crystallites in a graphite particle are typically oriented indifferent directions and, hence, a particular property of amulti-crystallite graphite particle is the directional average value ofall the constituent crystallites.

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known and the typical practice is described in U.S. Pat. No.3,404,061 to Shane et al., the disclosure of which is incorporatedherein by reference. In general, flakes of natural graphite (e.g. 100 inFIG. 1(b)) are intercalated in an acid solution to produce graphiteintercalation compounds (GICs, 102). The GICs are washed, dried, andthen exfoliated by exposure to a high temperature for a short period oftime. This causes the flakes to expand or exfoliate in the c-axisdirection of the graphite up to 80-300 times of their originaldimensions. The exfoliated graphite flakes are vermiform in appearanceand, hence, are commonly referred to as worms 104. These worms ofgraphite flakes which have been greatly expanded can be formed withoutthe use of a binder into cohesive or integrated sheets of expandedgraphite, e.g. webs, papers, strips, tapes, foils, mats or the like(typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications.

The upper left portion of FIG. 1(a) shows a flow chart that illustratesthe prior art processes used to fabricate flexible graphite foils andthe resin-impregnated flexible graphite composite. The processestypically begin with intercalating graphite particles 20 (e.g., naturalgraphite or synthetic graphite) with an intercalant (typically a strongacid or acid mixture) to obtain a graphite intercalation compound 22(GIC). After rinsing in water to remove excess acid, the GIC becomes“expandable graphite.” The GIC or expandable graphite is then exposed toa high temperature environment (e.g., in a tube furnace preset at atemperature in the range of 800-1,050° C.) for a short duration of time(typically from 15 seconds to 2 minutes). This thermal treatment allowsthe graphite to expand in its c-axis direction by a factor of 30 toseveral hundreds to obtain a worm-like vermicular structure 24 (graphiteworm), which contains exfoliated, but un-separated graphite flakes withlarge pores interposed between these interconnected flakes. An exampleof graphite worms is presented in FIG. 2(a).

In one prior art process, the exfoliated graphite (or mass of graphiteworms) is re-compressed by using a calendering or roll-pressingtechnique to obtain flexible graphite foils (26 in FIG. 1(a) or 106 inFIG. 1(b)), which are typically much thicker than 100 μm. An SEM imageof a cross-section of a flexible graphite foil is presented in FIG.2(b), which shows many graphite flakes with orientations not parallel tothe flexible graphite foil surface and there are many defects andimperfections.

Largely due to these mis-orientations of graphite flakes and thepresence of defects, commercially available flexible graphite foilsnormally have an in-plane electrical conductivity of 1,000-3,000 S/cm,through-plane (thickness-direction or Z-direction) electricalconductivity of 15-30 S/cm, in-plane thermal conductivity of 140-300W/mK, and through-plane thermal conductivity of approximately 10-30W/mK. These defects and mis-orientations are also responsible for thelow mechanical strength (e.g. defects are potential stress concentrationsites where cracks are preferentially initiated). These properties areinadequate for many thermal management applications and the presentinvention is made to address these issues.

In another prior art process, the exfoliated graphite worm 24 may beimpregnated with a resin and then compressed and cured to form aflexible graphite composite 28, which is normally of low strength aswell. In addition, upon resin impregnation, the electrical and thermalconductivity of the graphite worms could be reduced by two orders ofmagnitude.

The exfoliated graphite may be subjected to high-intensity mechanicalshearing/separation treatments using a high-intensity air jet mill,high-intensity ball mill, or ultrasonic device to produce separated nanographene platelets 33 (NGPs) with all the graphene platelets thinnerthan 100 nm, mostly thinner than 10 nm, and, in many cases, beingsingle-layer graphene (also illustrated as 112 in FIG. 1(b). An NGP iscomposed of a graphene sheet or a plurality of graphene sheets with eachsheet being a two-dimensional, hexagonal structure of carbon atoms.

With a low-intensity shearing, graphite worms tend to be separated intothe so-called “expanded graphite (108 in FIG. 19b )), which can beformed into graphite paper or mat 106 using a paper- or mat-makingprocess. This expanded graphite paper or mat 106 is just a simpleaggregate or stack of discrete flakes having defects, interruptions, andmis-orientations between these discrete flakes.

For the purpose of defining the geometry and orientation of an NGP, theNGP is described as having a length (the largest dimension), a width(the second largest dimension), and a thickness. The thickness is thesmallest dimension, which is no greater than 100 nm, preferably smallerthan 10 nm in the present application. When the platelet isapproximately circular in shape, the length and width are referred to asdiameter. In the presently defined NGPs, both the length and width canbe smaller than 1 μm, but can be larger than 200 μm.

A mass of multiple NGPs (including single-layer and/or few-layergraphene sheets, 33 in FIG. 1(a)) may be made into a graphene film/paper(34 in FIG. 1(a) or 114 in FIG. 1(b)) using a film- or paper-makingprocess. FIG. 3(b) shows a SEM image of a cross-section of a graphenepaper/film prepared from discrete graphene sheets using a paper-makingprocess. The image shows the presence of many discrete graphene sheetsbeing folded or interrupted (not integrated), most of plateletorientations being not parallel to the film/paper surface, the existenceof many defects or imperfections. NGP aggregates, even when beingclosely packed, exhibit a thermal conductivity higher than 1,000 W/mKonly when the film or paper is cast and strongly pressed into a sheethaving a thickness lower than 10 μm, and higher than 1,500 W/mK onlywhen the film or paper is cast and pressed into a sheet having athickness lower than 1 μm. A heat spreader in many electronic devices isnormally required to be thicker than 25 μm and, more desirably, thickerthan 50 μm based mainly on handling ease and structural integrityconsiderations (but no greater than 200 μm due to device volumeconstraint).

The precursor to the unitary graphene layer is graphene oxide (GO) gel.This gel is obtained by immersing a graphitic material 20 in a powder orfibrous form in a strong oxidizing liquid in a reaction vessel to form asuspension or slurry, which initially is optically opaque. This opticalopacity reflects the fact that, at the outset of the oxidizing reaction,the discrete graphite flakes and, at a later stage, the discretegraphene oxide flakes scatter visible wavelengths, resulting in anopaque and generally dark fluid mass. If the reaction between graphitepowder and the oxidizing agent is allowed to proceed at a sufficientlyhigh reaction temperature for a sufficient length of time, this opaquesuspension is transformed into a translucent or transparent solution,which is now a homogeneous fluid called “graphene oxide gel” (21 in FIG.1(a)) that contains no discernible discrete graphite flakes or graphiteoxide platelets.

In other words, this graphene oxide gel is optically transparent ortranslucent and visually homogeneous with no discernible discreteflakes/platelets of graphite, graphene, or graphene oxide dispersedtherein. In contrast, conventional suspension of discrete graphenesheets, graphene oxide sheets, and expanded graphite flakes look dark,black or heavy brown in color with individual graphene or graphene oxidesheets or expanded graphite flakes discernible or recognizable even withnaked eyes or a low-magnification light microscope (100×-1,000×).

The graphene oxide molecules dissolved in the liquid medium of agraphene oxide gel are aromatic chains that have an average number ofbenzene rings in the chain typically less than 1,000, more typicallyless than 500, and many less than 100. Most of the molecules have morethan 5 or 6 benzene rings (mostly >10 benzene rings) from combinedatomic force microscopy, high-resolution TEM, and molecular weightmeasurements. Based on our elemental analysis, these benzene-ring typeof aromatic molecules are heavily oxidized, containing a highconcentration of functional groups, such as —COOH and —OH and,therefore, are “soluble” (not just dispersible) in polar solvents, suchas water. The estimated molecular weight of these graphene oxidepolymers in the gel state is typically between 200 g/mole and 43,000g/mole, more typically between 400 g/mole and 21,500 g/mole, and mosttypically between 400 g/mole and 4,000 g/mole.

These soluble molecules behave like polymers and are surprisinglycapable of reacting and getting chemically connected with one another toform a unitary graphene layer of good structural integrity and highthermal conductivity. Conventional discrete graphene sheets, grapheneoxide sheets, or graphite flakes do not have any self-reacting orcohesive bonding capability.

Specifically and most significantly, these graphene oxide moleculespresent in a GO gel state are capable of chemically merging with oneanother and getting integrated into extremely long and wide graphenelayers (e.g. FIG. 3(a)) when the gel is dried and heat-treated at asufficiently high temperature for a sufficiently long period of time.These graphene layers can run as wide as the specimen width itself (upto hundreds of centimeters) that are parallel to one another. Noindividual graphene platelets or sheets are discernible; they have beenfully linked and integrated chemically with one another to form alayer-like unitary body. These unitary bodies appear to be chemicallybonded with one another along the thickness-direction (or Z-direction).X-ray diffraction studies have confirmed that the d-spacing(inter-graphene plane distance) has been recovered back to approximately0.335 nm (with <0.02% by weight of oxygen) to 0.40 nm (approximately5.0-10% oxygen). There does not appear to be any gap between thesegraphene layers and, hence, these layers have been essentially mergedinto one big unitary body, which is a graphene single crystal. FIG. 3(a)depicts an example of such a huge unitary body. Although there appearsto be some demarcations between unitary layers, these perceiveddemarcations are due to slightly different widths between layers. Eachlayer is composed of one of multiple graphene planes parallel to oneanother. These seemingly individual unitary layers actually have formedinto a single integrated entity or a graphene single crystal. Theformation process for such a graphene single crystal is furtherillustrated in FIG. 3(c).

It may be noted that the presently invented graphene single crystal isfundamentally different and patently distinct from the catalytic CVDgraphene thin film in terms of chemical composition, micro-structure,morphology, process of production, all chemical and physical properties,and intended applications:

-   -   (a) As schematically shown in FIG. 3(d), the prior art graphene        poly-crystal obtained by CVD of hydrocarbon on a catalytic        surface (e.g. Cu or Ni) is typically composed of many grains        with grain size typically smaller than 10 μm (most often <5 μm).        These grains also have different orientations with respect to        one another.    -   (b) In contrast, FIG. 3(e) shows a schematic of a graphene        single crystal of the present invention having just one single        grain or domain. There are no grain boundaries that can impede        the movement of electrons or phonons and, hence, this        single-grain single crystal has an exceptionally high electrical        conductivity and thermal conductivity.    -   (c) FIG. 3(f) shows a schematic of another graphene single        crystal of the present invention, which is a “poly-crystal” with        incomplete grain boundaries. The graphene planes in all the        grains are oriented parallel to one another.    -   (d) The presently invented graphene single crystal from GO gel        typically has an oxygen content from 0.01% to 5%, but no        hydrogen (H). In contrast, the catalytic CVD graphene film has        some hydrogen content, but no oxygen.    -   (e) Typically, the CVD graphene film grown on Cu or Ni surface        is single layer or inhomogeneous few-layer graphene with a        thickness less than 2 nm (the underlying Cu or Ni foil is not        capable of providing catalytic effect when the deposited carbon        layer exceeds 2 nm) These ultra-thin layers are thus optically        transparent and are intended for touch panel screen applications        to replace the ITO glass. In contrast, our graphene monolith is        typically thicker than 10 nm (more typically thicker than 1 μm,        and most typically thicker than 10 μm) and, hence, typically is        optically opaque. The graphene monolith of the present invention        has a significantly higher thermal conductivity and can be more        easily handled when being implemented into an electronic device        (e.g. a mobile phone) as a heat spreader.

The unitary graphene layer can be used alone as a heat spreader in anelectronic device. Alternatively, this unitary graphene layer can be acoating layer of a two-layer or multi-layer structure. In other words, alayer of graphene oxide-derived unitary graphene entity may be coatedonto one or two primary surfaces of a substrate or core layer of agraphitic material, forming a 2-layer or 3-layer structure. Thegraphitic core or substrate layer has two primary surfaces on theopposite sides of the layer. If one of the primary surfaces is coatedwith a layer of GO-derived unitary graphene, we have a 2-layer laminate.If both primary surfaces are coated with GO, we have a 3-layer laminate.One may further deposit a layer of protective material on a unitarygraphene coating layer to make a 4-layer laminate, for instance. Thisprotective layer can be an electrically insulating resin layer forcertain applications, e.g. for transferring heat from a CPU of a mobilephone or laptop computer to the GO coating layer so that the GO coatingcan help dissipate the heat generated by the CPU. The electricallyinsulating layer is essential to preventing internal shorting. Furtheroptionally, another layer of material (e.g. a thermal interfacematerial) can be deposited onto the opposite side of this 4-layerlaminate to make a 5-layer structure.

The unitary graphene-coated laminate preferably has a thickness nogreater than 1 mm, further preferably less than 200 μm, and mostpreferably less than 100 μm. More preferably, the thickness is greaterthan 10 μm, further preferably between 10 and 100 μm, and mostpreferably between 10 μm and 50 μm. A thickness less than 10 μm wouldmake it difficult to handle the laminate when attempting to incorporatepieces of the laminate in a device for thermal management applications(e.g. as a heat spreader in a microelectronic device).

In a special case of using graphene-based graphitic core layer, theconstituent graphene sheets (NGPs) preferably contain multi-layergraphene sheets preferably having a thickness of 3.35 nm to 33.5 nm.Preferably, the resulting graphitic core layer has a thickness nogreater than 100 μm, more preferably less than 50 μm. When multi-layergraphene sheets have a thickness of 6.7 nm to 20 nm, one can readilyproduce a graphitic core layer having an exceptional thermalconductivity.

The graphene-based graphitic core layer desirably contains pristinegraphene containing no oxygen. The pristine graphene can be obtainedfrom direct ultrasonication without involving oxidation of a graphiticmaterial. As shown in the upper portion of FIG. 1(a), pristine graphiteparticles 20 (without exposing to the oxidizing or other chemicaltreatment) may be directly exposed to high-intensity ultrasonication toproduce pristine graphene sheets. Multiple pristine graphene sheets maythen be aggregated together to form a graphene paper/film 38 via apaper-making procedure, for instance. The pristine graphene paper/film,as a graphitic core foil, is then coated with one or two layers of GOgel to obtain unitary graphene-coated laminate 42 after the heattreatment.

The GO coating material, when in a gel state, typically has an oxygencontent of 20-46% by weight. After being deposited onto a primarysurface of a graphitic core layer to form a laminate, the subsequentheat treatment process naturally reduces the oxygen content to typically0.01-10% by weight, more typically 0.01%-5%.

The graphene oxide is obtained from a graphene oxide gel, which gel iscomposed of graphene oxide molecules dispersed in an acidic mediumhaving a pH value of no higher than 5 and the graphene oxide moleculeshave an oxygen content no less than 20% by weight. In particular, thegel is obtained by immersing a graphitic material in a powder or fibrousform in an oxidizing liquid in a reaction vessel at a reactiontemperature for a length of time sufficient to obtain a graphene oxidegel composed of graphene oxide molecules dispersed in an acidic liquidmedium having a pH value of no higher than 5 and the graphene oxidemolecules have an oxygen content no less than 20% by weight. Asindicated in FIG. 1(a), the unitary graphene oxide coated laminate isformed by depositing a layer of graphene oxide gel 21 to one or bothprimary surfaces of a graphitic core layer 35 to form a GO gel-coatedgraphitic foil 36. By removing the residual liquid from the gel in theGO gel coating layer and subjecting the GO gel-coated laminate to a heattreatment we obtain the desired GO-coated graphitic foil laminate 40.

The starting graphitic material for the purpose of forming grapheneoxide gel may be selected from natural graphite, artificial graphite,meso-phase carbon, meso-phase pitch, meso-carbon micro-bead, softcarbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbonnano-tube, or a combination thereof. The graphitic material ispreferably in a powder or short filament form having a dimension lowerthan 20 μm, more preferably lower than 10 μm, further preferably smallerthan 5 μm, and most preferably smaller than 1 μm.

Using artificial graphite with an average particle size of 9.7 μm as anexample, a typical procedure involves dispersing graphite particles inan oxidizer mixture of sulfuric acid, nitric acid, and potassiumpermanganate (at a weight ratio of 3:1:0.05) at a temperature oftypically 0-60° C. for typically at least 3 days, preferably 5 days, andmore preferably 7 days or longer. The average molecular weight of theresulting graphene oxide molecules in a gel is approximately20,000-40,000 g/mole if the treatment time is 3 days, <10,000 g/mole if5 days, and <4,000 g/mole if longer than 7 days. The required gelformation time is dependent upon the particle size of the originalgraphitic material, a smaller size requiring a shorter time. It is offundamental significance to note that if the critical gel formation timeis not reached, the suspension of graphite powder and oxidizer (graphiteparticles dispersed in the oxidizer liquid) appears completely opaque,meaning that discrete graphite particles or flakes remain suspended (butnot dissolved) in the liquid medium. As soon as this critical time isexceeded, the whole suspension becomes optically translucent ortransparent, meaning that the heavily oxidized graphite completely losesits original graphite identity and the resulting graphene oxidemolecules are completely dissolved in the oxidizer liquid, forming ahomogeneous solution (no longer just a suspension or slurry).

It must be further noted that if the suspension or slurry, with atreatment time being shorter than the required gel formation time, isrinsed and dried, we would simply recover a graphite oxide powder orgraphite intercalation compound (GIC) powder, which can be exfoliatedand separated to produce nano graphene platelets (NGPs). Without anadequate amount of a strong oxidizing agent and an adequate duration ofoxidation time, the graphite or graphite oxide particles would not beconverted into the GO gel state.

Hence, the NGPs (for use in a graphitic core layer) may be produced bysubjecting a graphitic material to a combined treatment of oxidation,exfoliation, and separation. This graphitic material may also beselected from natural graphite, artificial graphite, meso-phase carbon,meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon,coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof. The NGPs can also be produced from a process suchas (a) direct ultrasonication, (b) potassium melt intercalation andwater/alcohol-induced exfoliation, or (c) supercritical fluidintercalation/exfoliation/separation of non-oxidized graphitic material.These processes produce pristine graphene that contains no oxygen.

The graphene oxide-derived unitary graphene-coated laminate of thepresent invention typically has a thermal conductivity greater than 800W/mK, more typically greater than 1,000 W/mK (even when the filmthickness is greater than 10 μm) and often greater than 1,700 W/mK. Thislatter valve is typically obtained when the graphitic core layer isrelatively thin compared to the unitary graphene coating layers and whenthe final heat treatment temperature is higher than 2,500° C. The coatedlaminate typically has an electrical conductivity greater than 3,000S/cm (even >10,000 S/cm). This high electrical conductivity (greaterthan 3000 S/cm and up to 20,000 S/cm) can be achieved concurrently witha thermal conductivity greater than 1,000 W/mK (up to 1,900 W/mK). Quiteoften, the unitary graphene-coated laminate can exhibit a combination ofa high electrical conductivity (greater than 1,500 S/cm), a high thermalconductivity (greater than 600 W/mK), a relatively high physical density(greater than 1.4 g/cm³), and a relatively high tensile strength(greater than 10 MPa, often >40 MPa, and can be >120 MPa). The unitarygraphene layer-coated laminates also exhibit an exceptional surfacehardness and scratch resistance, eliminating the tendency for agraphitic core foil (particularly flexible graphite foil andrecompressed graphene platelet foil) to flake of (to emit free carbon orgraphite particles into air).

Quite surprisingly, in many samples, the unitary graphene layer-coatedlaminate has an electrical conductivity greater than 2,000 S/cm, athermal conductivity greater than 800 W/mK, a physical density greaterthan 1.8 g/cm³, and a tensile strength greater than 40 MPa. Thiscombination of superior properties has not been achieved with anygraphite or non-graphite material. In some cases, the coated laminateexhibits an electrical conductivity greater than 3,000 S/cm (up to20,000 S/cm), a thermal conductivity greater than 1,500 W/mK (up to1,900 W/mK), a physical density greater than 2.0 g/cm³, and a tensilestrength greater than 40 MPa (up to 120 MPa). This type of grapheneoxide-coated laminate may be used as a heat spreader component in aportable device.

The present invention also provides a process for producing a unitarygraphene layer or graphene single crystal. The process comprises: (a)preparing a graphene oxide gel having graphene oxide molecules dispersedin a fluid medium, wherein the graphene oxide gel is opticallytransparent or translucent; (b) depositing a layer of the graphene oxidegel onto a surface of a supporting substrate to form a depositedgraphene oxide gel thereon; (c) partially or completely removing thefluid medium from the deposited graphene oxide gel layer to form agraphene oxide layer; and (d) heat-treating the graphene oxide layer toform the unitary graphene layer or graphene single crystal. The processmay advantageously further comprise a step of compressing the grapheneoxide layer before, during, and/or after the heat-treating step.

The graphene oxide gel is prepared by immersing a graphitic material ina powder or fibrous form in an oxidizing liquid to form an initiallyoptically opaque suspension in a reaction vessel at a reactiontemperature for a length of time sufficient to obtain a graphene oxidegel that is optically transparent or translucent, wherein the grapheneoxide gel is composed of graphene oxide molecules dispersed in an acidicmedium having a pH value of no higher than 5 and the graphene oxidemolecules have an oxygen content no less than 20% by weight. Thestarting material for the preparation of GO gel is a graphitic materialselected from natural graphite, artificial graphite, meso-phase carbon,meso-phase pitch, meso-carbon micro-bead, soft carbon, hard carbon,coke, carbon fiber, carbon nano-fiber, carbon nano-tube, or acombination thereof.

In the process, steps (b) and (c) may advantageously include feeding asheet of a solid substrate material from a roller to a deposition zone,depositing a layer of graphene oxide gel onto a surface of the sheet ofsolid substrate material to form a graphene oxide gel layer thereon,drying the graphene oxide gel to form a dried graphene oxide layerdeposited on the substrate surface, and collecting graphene oxidelayer-deposited substrate sheet on a collector roller. This isessentially a roll-to-roll process that is amenable to continuous massproduction of unitary graphene materials. The process may furthercomprise a step of compressing the graphene oxide layer prior to beingcollected on the collector roller.

In the process, the graphene oxide gel may be deposited onto a primarysurface of a supporting substrate using any coating, casting, spraying,or liquid-dispensing process. Upon removal of the liquid medium from thecoating layer, the resulting coated laminate is then subjected to athermal treatment or re-graphitization treatment (typically 100-1000°C., but can be higher), which allows individual graphene oxide moleculesto chemically bond to one another. This thermal treatment surprisinglyenables or activates the re-joining, polymerization, or chain-growth ofotherwise small graphene oxide molecules, resulting in removal ofnon-carbon elements (e.g. all H and most O) and formation of hugegraphene sheets. It appears that the graphene oxide molecules can bemerged and integrated into several unitary graphene layers that runparallel to one another and these graphene layers can cover the entirelength of the coating layer without interruption. In other words, thesegraphene layers are each a complete unitary graphene entity. Thesecomplete unitary graphene layers actually constitute one unitary entitythat is essentially a graphene block with all graphene planes beingoriented along one single direction (e.g. as schematically shown in FIG.3(a) or 3(c)).

The unitary graphene layer of the present invention is often a singlecrystal (as schematically shown in FIG. 3(e)) or a poly-crystal withincomplete grain boundaries (e.g. schematically shown in FIG. 3(f))which is essentially a graphene single crystal as well. By contrast, theprior art graphene film (single layer or few layer <2 nm thick) preparedby catalytic chemical vapor deposition (CVD) on a catalyst surface (Cuor Ni) is essentially poly-crystalline graphene with grain sizestypically <100 μm and more typically <10 μm. This CVD graphene film isintended for use as a semiconductor material (e.g. to replace Si in aFET transistor) or as a touch panel screen (e.g. to replace ITO glassused in a display device such as mobile phone screen). This CVD grapheneis made by catalyst-assisted decomposition of hydrocarbon gas moleculesand deposition of resulting carbon atoms on a Cu or Ni foil at a CVDtemperature of typically 800-1,000° C. The electrical conductivity(<2,000 S/cm) and thermal conductivity (<500 W/mK) of the CVD graphenefilms are typically significantly lower than those of the presentlyinvented graphene single crystals even though these CVD films aretypically thinner than 2 nm and our graphene single crystals aretypically thicker than 10 nm (often thicker than 10 μm).

This unitary body (or “single crystal”) of highly oriented grapheneplanes exhibits an unprecedented combination of exceptional thermalconductivity, electrical conductivity, structural integrity (strengthand ease of handling). These properties are unmatched by any graphiticor non-graphitic material.

The thermal treatment process can be assisted with a calendering orroll-pressing operation to help improve the surface finish of theresulting coated laminate. The unitary graphene layer thickness can beless than 10 μm, but preferably between 10 μm and 200 μm, and mostpreferably between 20 μm and 100 μm.

As indicated above, flexible graphite foils prepared by re-compressionof exfoliated graphite flakes or graphite worms exhibit relatively lowthermal conductivity and mechanical strength. The graphite worms can beformed into flexible graphite foils by compression, without the use ofany binding material, presumably due to the mechanical interlockingbetween the voluminously expanded graphite flakes. Although asignificant proportion of these flakes are oriented in a directionlargely parallel to the opposing surfaces of a flexible graphite sheet(as evidenced by the high degree of anisotropy with respect to thermaland electrical conductivity), many other flakes are distorted, kinked,bent over, or oriented in a direction non-parallel to these sheetsurfaces (FIG. 2(b)). This observation has been well demonstrated inmany scanning electron micrographs (SEM) published in open or patentliterature. Furthermore, the presence of a large number of graphiteflakes implies a large amount of interface between flakes, resulting invery high contact resistance (both thermal and electrical resistance).

As a consequence, the electrical or thermal conductivity of theresulting flexible graphite foils dramatically deviates from what wouldbe expected of a perfect graphite single crystal or a graphene layer.For instance, the theoretical in-plane electrical conductivity andthermal conductivity of a graphene layer are predicted to be 1-5×10⁴S/cm and 3,000-5,000 W/(mK), respectively. However, the actualcorresponding values for flexible graphite foils are 1-3×10³ S/cm and140-300 W/(mK), respectively; one order of magnitude lower than whatcould be achieved. By contrast, the corresponding values for thepresently invented unitary graphene-coated graphitic foil areapproximately 3.5-20×10³ S/cm (3,500-20,000 S/cm) and 600-2,000 W/(mK),respectively.

The present invention also provides a highly thermally conductiveunitary graphene layer or unitary graphene-coated laminate that can beused for thermal management applications; e.g. for use as a heatspreader in a microelectronic device (such as mobile phone, notebookcomputer, e-book, and tablet), flexible display, light-emitting diode(LED), power tool, computer CPU, and power electronics. We are filingseparate patent applications to claim the various products orapplications of the presently invented GO-coated graphitic laminates.

Example 1 Preparation of Nano Graphene Platelets (NGPs)

Chopped graphite fibers with an average diameter of 12 μm was used as astarting material, which was immersed in a mixture of concentratedsulfuric acid, nitric acid, and potassium permanganate (as the chemicalintercalate and oxidizer) to prepare graphite intercalation compounds(GICs). The fiber segments were first dried in a vacuum oven for 24 h at80° C. Then, a mixture of concentrated sulfuric acid, fuming nitricacid, and potassium permanganate (at a weight ratio of 4:1:0.05) wasslowly added, under appropriate cooling and stirring, to a three-neckflask containing fiber segments. After 16 hours of reaction, theacid-treated graphite fibers were filtered and washed thoroughly withdeionized water until the pH level of the solution reached 6. Afterbeing dried at 100° C. overnight, the resulting graphite intercalationcompound (GIC) was subjected to a thermal shock at 1050° C. for 45seconds in a tube furnace to form exfoliated graphite (worms). Fivegrams of the resulting exfoliated graphite (EG) were mixed with 2,000 mlalcohol solution consisting of alcohol and distilled water with a ratioof 65:35 for 12 hours to obtain a suspension. Then the mixture orsuspension was subjected to ultrasonic irradiation with a power of 200 Wfor various times. After two hours of sonication, EG particles wereeffectively fragmented into thin NGPs. The suspension was then filteredand dried at 80° C. to remove residue solvents. The as-prepared NGPshave an average thickness of approximately 9.7 nm.

Example 2 Preparation of Single-Layer Graphene from Meso-CarbonMicro-Beads (MCMBs)

Meso-carbon microbeads (MCMBs) were supplied from China Steel ChemicalCo. This material has a density of about 2.24 g/cm³ with a medianparticle size of about 16 μm. MCMB (10 grams) were intercalated with anacid solution (sulfuric acid, nitric acid, and potassium permanganate ata ratio of 4:1:0.05) for 72 hours. Upon completion of the reaction, themixture was poured into deionized water and filtered. The intercalatedMCMBs were repeatedly washed in a 5% solution of HCl to remove most ofthe sulphate ions. The sample was then washed repeatedly with deionizedwater until the pH of the filtrate was neutral. The slurry was dried andstored in a vacuum oven at 60° C. for 24 hours. The dried powder samplewas placed in a quartz tube and inserted into a horizontal tube furnacepre-set at a desired temperature, 1,080° C. for 45 seconds to obtain agraphene material. TEM and atomic force microscopic studies indicatethat most of the NGPs were single-layer graphene.

Example 3 Preparation of Pristine Graphene

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours.

Example 4 Preparation of Graphene Oxide (GO) Gel

Graphite oxide gel was prepared by oxidation of graphite flakes with anoxidizer liquid consisting of sulfuric acid, sodium nitrate, andpotassium permanganate at a ratio of 4:1:0.05 at 30° C. When naturalgraphite flakes (particle sizes of 14 μm) were immersed and dispersed inthe oxidizer mixture liquid, the suspension or slurry appears opticallyopaque and dark. The suspension remains opaque during the first 52 hoursof reaction. However, the suspension gradually turns opticallytranslucent (a little cloudy) when the reaction time exceeds 52 hours,and the color of the suspension changes from black to dark brown. After96 hours, the suspension suddenly becomes an optically transparentsolution with light brown color. The solution appears very uniform incolor and transparency, indicating the absence of any dispersed discreteobjects. The whole solution behaves like a gel, very similar to atypical polymer gel.

Surprisingly, by casting this gel on a glass surface and removing theliquid medium from the cast film we obtain a thin film of graphene oxidethat is optically transparent. This thin film looks like, feels like,and behaves like a regular polymer film. However, upon re-graphitizationat a temperature (typically >100° C., more typically >1,000° C., furthertypically >1,500° C., and can be >2,500° C.) for typically 1-3 hours,this GO film is transformed in a unitary graphene entity comprising orbeing a large-size graphene single crystal. This is a free-standingunitary graphene layer, which can be implemented directly as a heatspreader in an electronic device. Alternatively, as one of the manyapplications, a unitary graphene layer may be deposited onto a surfaceof a graphitic substrate layer to form a thermally conductive laminate,described below:

By casting a layer of GO gel onto a graphitic material foil (graphitepaper, film, mat, etc) and removing the liquid content from the gel, theresulting solid coating layer sticks to the graphitic foil very well.Actually, the bonding between the GO coating and the graphitic foil isvery strong and, after a heat treatment, the GO-derived unitary graphenelayer cannot be peeled off from the graphitic foil without tearing apartor removing some graphitic foil material. This implies that GO gel, uponre-graphitization, possesses an exceptional cohesive strength. This is avery surprising observation.

X-ray diffraction curves of a GO film (GO gel coated on a glass surfacewith liquid medium removed), a GO film thermally reduced at 150° C. forone hour, and a highly reduced and re-graphitized GO film (a unitarygraphene layer) are shown in FIGS. 5(a), 5(b), and 5(c), respectively.The peak at approximately 2θ=12° of the dried GO film (FIG. 5(a))corresponds to an inter-graphene spacing (d₀₀₂) of approximately 0.7 nm.With some heat treatment at 150° C., the GO film exhibits the formationof a hump centered at 22° (FIG. 5(b)), indicating that it has begun theprocess of decreasing the inter-graphene spacing, indicating a partialrecovery of the original structure of natural graphite particles. With aheat treatment temperature of 2,500° C. for one hour, the d₀₀₂ spacinghas decreased to approximately 0.336, close to 0.335 nm of naturalgraphite.

The inter-graphene spacing values of GO-derived unitary graphene filmsobtained by heat treating at various temperatures over a widetemperature range are summarized in FIG. 6(a). Corresponding oxygencontent values in the GO-derived unitary graphene layer are shown inFIG. 6(b). The thermal conductivity of GO-derived unitary graphene layerand corresponding flexible graphite (FG) foil, also plotted as afunction of the same final heat treatment temperature range issummarized in FIG. 6(c). It is of significance to point out that a heattreatment temperature as low as 500° C. is sufficient to bring theaverage inter-graphene spacing in GO back to below 0.4 nm, gettingcloser and closer to that of natural graphite. The beauty of thisapproach is the notion that this GO gel strategy has enabled us tore-organize and re-orient the graphene planes of carbon atoms into agraphene monolith with all the graphene planes being now essentiallyparallel to one another. This has given rise to a thermal conductivityalready >420 W/mK, which is more than twice of the value (200 W/mK) ofthe corresponding flexible graphite foil. These graphene planes ofcarbon atoms are derived from the graphene planes that constitute theoriginal natural graphite structure. The original natural graphiteparticles, when randomly packed into an aggregate or “graphite compact”,have their constituent graphene planes randomly oriented, exhibitrelatively low thermal conductivity, and have essentially zero strength(no structural integrity). In contrast, the strength of the unitarygraphene layer is typically in the range of 40-140 MPa.

With a heat treatment temperature as low as 800° C., the resultingunitary graphene layer exhibits a thermal conductivity of 1,148 W/mK, incontrast to the 244 W/mK of the flexible graphite foil with an identicalheat treatment temperature. As a matter of fact, no matter how high theheat treatment temperature is (e.g. even as high as 2,800° C.), theflexible graphite foil only shows a thermal conductivity lower than 600W/mK. At a heat treatment temperature of 2,800° C., the presentlyinvented unitary graphene layer delivers a thermal conductivity of 1,807W/mK.

A close scrutiny and comparison of FIGS. 2(a), 3(a), and 3(b) indicatesthat the graphene layers in a graphene single crystal or graphenemonolithic are substantially oriented parallel to one another; but thisis not the case for flexible graphite foils and graphene oxide paper.The inclination angles between two identifiable layers in the unitarygraphene entity are mostly less than 5 degrees. In contrast, there areso many folded graphite flakes, kinks, and mis-orientations in flexiblegraphite that many of the angles between two graphite flakes are greaterthan 10 degrees, some as high as 45 degrees (FIG. 2(b)). Although notnearly as bad, the mis-orientations between graphene platelets in NGPpaper (FIG. 3(b)) are also high and there are many gaps betweenplatelets. The unitary graphene entity is essentially gap-free.

FIG. 4 (a) shows the thermal conductivity values of the GO-derivedunitary graphene layer (▴), GO paper (▪) prepared by vacuum-assistedfiltration of RGO, and FG foil (♦), respectively, all plotted as afunction of the final heat treatment temperature for graphitization orre-graphitization. These data have clearly demonstrated the superiorityof the unitary graphene layer or graphene single crystal in terms of theachievable thermal conductivity at a given heat treatment temperature.

For comparison, we also carbonized polyimide films at 500° C. for 1 hourand at 1,000° C. for 3 hours in an inert atmosphere and then graphitizedthe films at a temperature in the range of 2,500-3,000° C. for 1 to 5hours to form a conventional graphitic film, pyrolytic graphite (PG).FIG. 4(b) shows the thermal conductivity values of the GO-derivedunitary graphene layer (▪) and the polyimide-derived pyrolytic graphite(PG) heat-treated for one hour (x) and for 3 hours (▴), all plotted as afunction of the final graphitization or re-graphitization temperature.These data show that the conventional pyrolytic graphite (PG), producedby carbonizing polyimide and then graphitizing the carbonized PI,exhibits a consistently lower thermal conductivity as compared to theGO-derived unitary graphene layer alone (▪), given the same heattreatment (graphitization or re-graphitization) temperature for the samelength of heat treatment time. For instance, the PG from PI exhibits athermal conductivity of 820 W/mK after a graphitization treatment at2,000° C. for one hour and 1,242 W/mK at 2,000° C. for 3 hours. Theseobservations have demonstrated a clear and significant advantage ofusing the GO gel approach versus the conventional PG approach. As amatter of fact, no matter how long the graphitization time is for thePG, the thermal conductivity is always lower than that of a GOgel-derived unitary graphene layer. In other words, the unitary graphenelayer is fundamentally different and patently distinct from both theflexible graphite and the pyrolytic graphite in terms of chemicalcomposition, structure, morphology, process of production, andproperties.

The above conclusion is further supported by the data in FIG. 4(c)showing the electric conductivity values of the GO-derived unitarygraphene layer (♦) are far superior to those of the GO paper (▪) fromRGO platelets and FG foil (x) over the entire range of finalgraphitization or re-graphitization temperatures investigated.

Examples 5 Preparation and Testing of Laminates with a GrapheneOxide-Derived Unitary Graphene Layer Coated on a Core or SubstrateGraphitic Foil

The NGPs prepared in Examples 1-3 were made into thin paper or film form(e.g., using a vacuum-assisted filtration technique) for use as agraphitic foil. Other graphitic foils investigated include carbonnano-tube paper (Bucky paper from Buckeye Composites, Inc., Dayton,Ohio), carbon nano-fiber mat (CNFs supplied from Applied Sciences, Inc.,Cedarville, Ohio), flexible graphite foils of several differentthicknesses (supplied from Graftech and Timcal Graphite), graphite-epoxycomposite films prepared in-house, carbon-carbon composite layerprepared in-house (carbon fiber mat reinforced with phenolic resin,which was then carbonized), and carbon paper (Toray). We have alsoprepared CNT and graphene films that are nanometers in thickness. Thesenano-thin films were deposited on a plastic film first (PET film,polyethylene terephthalate). PET film serves as an electricallyinsulating layer to electrically isolate the heat spreader layer from aCPU of a mobile phone or laptop computer.

The graphene oxide gel prepared in Example 4 was used for thepreparation of graphene oxide-derived unitary graphene coating. In somecases, discrete NGPs were added into the GO gel to form a mixture gel orNGP-GO gel slurry. In these cases, fully separated NGP platelets weredispersed in graphene oxide gel to produce a graphene platelet gel withthe platelet concentration of approximately 1-50% by weight (preferably5-20% by weight NGP). Ultrasonic waves were employed to assist in thedispersion of NGPs in the gel.

Pure GO gel or NGP-GO mixture gel or slurry was then cast onto a primarysurface or both primary surfaces of a graphitic foil using a coatingmachine equipped with drying and heating provisions. In some cases, theGO gel or NGP-GO gel mixture was casted onto a substrate and regulatedby a doctor's blade to form a uniform coating thereon. The liquid in thecoating was further removed in a vacuum oven to form a solid GO coating.The resulting coated graphitic foils (also referred to as coatedlaminates) were then subjected to a heat treatment at a temperature offrom 100° C., up to approximately 3,000° C. We have utilized severaltemperature regimes: 100° C.-1,000° C.; 1,000° C.-1,500° C.; 1,500°C.-2,500° C.; and 2,500° C.-3,000° C.

Examples 6 Electrical and Thermal Conductivity Measurements of VariousGraphene Oxide-Derived Unitary Graphene-Coated Graphitic Foils

Four-point probe tests were conducted on unitary graphene layer-coatedgraphitic foils (e.g. flexible graphite foil, FG), the GO-derivedunitary graphene layer alone (coated on a glass surface and then peeledoff and heat treated), and the FG foils alone (no coating) to measuretheir in-plane electrical conductivity. Their in-plane thermalconductivity was measured using a laser flash method (Netzsch ThermalDiffusivity Device).

The in-plane thermal and electrical conductivities and tensileproperties of various films or laminates were investigated. Severalsignificant observations can be made from the testing results (e.g. assummarized in FIG. 8(a), (b), and (c):

-   -   (1) With a thickness of approximately 75 μm, the thermal        conductivity of the flexible graphite foil alone (FG, ▴ in FIG.        4(a)) is less than 237 W/mK if the FG foil is not heat-treated        at or above 700° C. As the post-recompression heat treatment        temperature increases from 700° C. to 2,800° C. (for one hour of        graphitization treatment in each case), the thermal conductivity        of the FG foil increases from 237 to 582 W/mK, indicating some        but limited re-organization of the graphitic structure induced        by the heat treatment. By contrast, the thermal conductivity of        the GO-derived unitary graphene layer alone (obtained by        depositing a layer of GO gel on a glass surface, removing the        liquid from the GO layer in vacuum for 1 hour, and peeling off        the dried solid GO layer from the glass surface) increases from        983 to 1,807 W/mK (▪ in FIG. 8(a)). This indicates a significant        or dramatic re-organization of the graphitic structure induced        by the heat treatment, with all GO molecules linked or merged        edge-to-edge into unitary graphene layers and integrated into a        unitary entity of fully and orderly bonded graphene planes, a        graphene single crystal.    -   (2) The thermal conductivity of a corresponding series of        GO-derived unitary graphene-coated FG foils (♦ in FIG. 8(a))        increases from 698 to 1,803 W/mK. This is significantly higher        than the thermal conductivity values of what would be        theoretically predicted (x in FIG. 8a )) from a rule-of-mixture        law, which is commonly used to predict composite properties from        constituent properties. Each coating layer is approximately 75        μm thick. These data have clearly demonstrated an un-expected,        synergistic effect between GO-derived unitary graphene coating        (derived from graphene oxide gel) and the FG foil core layer.    -   (3) FIG. 8(b) shows that the conventional pyrolytic graphite        (PG), produced by carbonizing polyimide and then graphitizing        the carbonized PI, exhibits a consistently lower thermal        conductivity as compared to the GO-derived unitary graphene        layer alone (▪) or unitary graphene layer-coated FG laminate        (♦), given the same heat treatment (graphitization or        re-graphitization) temperature for the same length of heat        treatment time. For instance, the PG from PI exhibits a thermal        conductivity of 820 W/mK after a graphitization treatment at        2,000° C. for one hour and 1,242 W/mK at 2,000° C. for 3 hours.        These observations have demonstrated a clear and significant        advantage of using the GO gel approach versus the conventional        PG approach. As a matter of fact, no matter how long the        graphitization time is for the PG, the thermal conductivity is        always lower than that of a GO gel-derived unitary graphene or        unitary graphene-coated FG laminate. In other words, both the        GO-derived unitary graphene layer and unitary graphene-coated        graphitic foils are fundamentally different and patently        distinct from the pyrolytic graphite in terms of chemical        composition, structure, morphology, process of production, and        properties.    -   (4) FIG. 8(c) also shows a dramatic synergistic effect in        electrical conductivity when GO gel is coated to the surfaces of        a FG foil to form a GO-coated laminate and then heat-treated to        form the unitary graphene-coated laminate. All the electric        conductivity values of unitary graphene-coated laminates are        significantly higher than those predicted by the rule-of-mixture        law.

Examples 7 Tensile Strength of Various Graphene Oxide-Derived UnitaryGraphene-Coated Graphitic Foils

A series of GO-derived unitary graphene-coated FG foils were preparedwith the coating thickness varying from approximately 10 μm to 80 μm percoating layer. The core FG foil is approximately 100 μm thick. Auniversal testing machine was used to determine the tensile strength ofvarious coated laminates and their uncoated counterparts (core layeronly).

The tensile strength values are plotted as a function of thecoating-to-core thickness ratio, FIG. 9(a). The data have demonstratedthat the tensile strength of the flexible graphite foil increasesmonotonically with respect to the GO-derived unitary graphene coatingthickness. (The effect of the specimen thickness increases, due to thedeposition of GO-derived unitary graphene layers, has been taken intoaccount by dividing the specimen breaking force by the actual specimencross-sectional area.) A ten-fold increase in tensile strength, from 12MPa to 121 MPa, was observed with two coating layers (each 80 μm thick)being uniformly deposited onto the two primary surfaces of the FG foil.This result is quite striking and further reflects the notion that theGO gel-derived GO layer (a unitary graphene entity or graphene singlecrystal) is a class of material by itself.

SEM investigation of specimen fracture surfaces has shown that thefailure of these foils and laminates was always initiated from the FGfoil itself (not from a GO-derived unitary graphene coating layer, ifpresent), and often from a site near a FG surface or edge. The FGsurface or edge appears to have abundant surface defects that couldserve as a mechanical stress concentration site, promoting crackinitiation. The presence of a unitary graphene coating layer appears tobe capable of significantly smoothing out the surface defects anddelaying the crack initiation step. When the stress is sufficientlyhigh, cracks are eventually initiated near the FG surface/edge, leadingto final tensile failure.

Another unexpected observation is the notion that the tensile strengthof the unitary graphene-coated laminate increases monotonically with theincreasing unitary graphene coating amount, reaching a value of 121 MPathat is one order of magnitude higher than the typical strength offlexible graphite-type materials. This appears to suggest that GO gelhas a strong adhering power capable of bonding GO to a graphitic foiland also that GO molecules in a GO gel are capable ofcombining/integrating with one another to form larger and strongerunitary graphene layers or single crystals that are relativelydefect-free, leading to a relatively high cohesive strength.

Examples 8 The Surface Scratch Resistance (in Terms of ScratchVisibility and Scratch Depth), and Hardness of Various GrapheneOxide-Coated Graphitic Foils

The scratch test was conducted using the so-called Ford Lab Test Method(FLTM) BN108-13. This apparatus consists of a movable platform connectedto five beams with 250 mm in length. A scratch pin is attached to oneend of each beam. A highly polished hardened steel ball (1.0±0.1 mmdiameter) is placed on the tip of each pin. Each pin is loaded with aweight that exerts a force of 7N, 6N, 3N, 2N, and 0.6N, respectively.Driven by compressed air, the beams draw the pins across the specimensurface and generate scratches. The scratch is made at a slidingvelocity of approximately 100 mm/s. All tests were performed at roomtemperature. Although the test method requires that grained surfaces beevaluated, only the smooth surfaces of the specimens were tested in thisstudy.

After the specimen plaques were scratched, they were evaluated with areflected light polarizing microscope incorporating a Xenon lightsource. An image analyzer with Image Analysis Software was used tomeasure the “gray scale mass,” which is the total gray scale value ofthe object. The camera objective lens is positioned at an angle of 90°from the scratch. The objective lens then registers a portion of thescratch about 1 mm long. The electron signal for each scratch line isthen integrated and recorded. The optical mass of an object, M, is thesum of the gray level values, GL, of all pixels in the object. Theindividual gray level values are assigned by the image analysis programin unit steps in the range of 0-255, where 0=black and 255=white. Theoptical mass, M, can be computed from: M=ΣGL_(i) (sum over i to n),where n is the number of pixels. The brightness of the object, B, isB=M/A, where A represents the area of the object. The percentage changein the brightness between the scratch and the background is the scratchvisibility, ΔB, given byΔB=[(B_(scratch)−B_(background))/(B_(background))]×100%. The depth ofthe scratch was measured using an interferometer. The magnification wasset at 5×. Depth measurements were made from the depth histogram of thescanned area. The scratches were also examined using a scanning electronmicroscope (SEM).

Indentation hardness tests were also performed on selected specimens.For the Rockwell Hardness test, the ASTM D 785 test procedure wasfollowed. The indenter was a round steel ball with 12.5 mm in diameter(Rockwell R scale). The Rockwell hardness number is a measure of thenon-recoverable indentation after a heavy load of 588N for a period of15 s, and subsequently reduced to a minor load of 98N for anotherduration of 15 s. Normal hardness is then defined as the load divided bythe projected area.

FIGS. 9(b), 9(c), and 9(d) show the scratch visibility, scratch depth,and Rockwell hardness data, respectively, of a series of GO-derivedunitary graphene-coated FG foils plotted as a function of thecoating-to-core layer thickness ratio. These data indicate that the bareFG foils are not scratch resistant, exhibiting a high level of scratchvisibility and deep scratch marks. The scratch resistance issignificantly improved by depositing a thin layer of GO-derived unitarygraphene coating; the thicker the unitary graphene coating, the betterthe scratch resistance. This scratch resistance is a reflection ofimproved surface hardness due to the unitary graphene coating layer asshown in FIG. 9(d).

Examples 9 Properties of Various Types of Unitary Graphene-CoatedGraphitic Foils

Examples of graphitic foils investigated include: NGP paper, CNT paper,CNF mat, graphite-epoxy composite films, carbon-carbon composite layer,carbon paper, GO gel-bonded NGP composite, PET-supported CNT film, andPET-supported graphene film (PET film was attached after heat treatingGO coating). The physical and mechanical properties of a broad array ofGO-coated graphitic foils are summarized in Table 1 below. Theimprovements in various properties over the core or substrate alone aredue to the superior properties of the GO gel-derived unitary graphenelayer or the synergistic effects between the GO gel-derived unitarygraphene layer and the core/substrate layer. These exceptionalproperties are not observed with the core layer alone (e.g. flexiblegraphite), the catalytic CVD graphene film, and the pyrolytic graphite(including HOPG).

TABLE 1 Properties of selected graphitic foils and their GO-derivedunitary graphene-coated versions. Coating Graphitic core heat-treatingElect. Thermal Tensile Surface Sample layer & temp. and Cond. cond.strength hardness Scratch No. thickness thickness (S/cm) (W/mk) (MPa)(Rockwell) resistance 20-A NGP paper, 120 μm None 1427 354 27 34 Fair20-B NGP paper, 120 μm 1,500° C.; 40 μm 4,205 765 98 72 Excellent 21-ACNT paper, 126 μm None 238 67 61 26 Fair 21-B CNT paper, 126 μm 1,500°C.; 40 μm 4,122 667 235 88 Excellent 22-A CNF mat, 155 μm None 162 54 5228 Fair 22-B CNF mat, 155 μm 1,500° C.; 40 μm 3825 621 167 82 Excellent23-A Graphite- None 83 11 — — — epoxy, 268 μm 23-B Graphite- 300° C.; 65μm 1224 252 — — — epoxy, 268 μm 24-A C/C composite, None 320 88 — — —232 μm 24-B C/C composite, 1,200° C.; 45 μm 3672 617 — — — 232 μm 25-ACarbon paper, None 167 61 62 25 Poor 156 μm 25-B Carbon paper, 2,200°C.; 75 μm 5,250 970 165 82 Excellent 156 μm 26-A GO gel-bonded None3,241 602 — — — NGP composite 103 μm 26-B GO gel-bonded 1,500° C.; 40 μm4,725 898 — — — NGP composite 103 μm

These data have further demonstrated that a GO-derived unitary graphenecoating can significantly improve electric conductivity, thermalconductivity, tensile strength, scratch resistance, and hardness of agraphitic layer. The excellent scratch resistance and surface hardnessalso result in the reduction or elimination of any graphite or carbonparticles from being scratched off the foil surface.

A comparison between Sample 21-A and 21-B and that between 22-A and 22-Breveal a very interesting and significant result. The paper or mat madefrom CNTs and CNFs, two presumably high-strength materials, showrelatively poor strength and hardness. Presumably one could impregnatethese porous paper or mat structures with a resin, but that woulddramatically curtail the electrical conductivity and thermalconductivity. Instead, by coating both major surfaces of a mat or paperstructure with a thin layer of GO gel and then drying andre-graphitizing the GO layer at 1,000° C. for one hour, we have observedthat the tensile strengths of both materials were increased by a factorof 3-4. Furthermore, both the thermal conductivity and the electricalconductivity of both CNT paper and CNF mat were increased by one orderof magnitude. These exceptional properties, alone or in combination,could not have been achieved with resin-impregnated CNT paper or CNF mateven if the resin matrix is carbonized at 1,500° C. for an extendedperiod of time. It is well-known in the art that carbon/carboncomposites, even after repeated resin impregnations and carbonizations,could not reach an electrical conductivity of 1,000 S/cm (typically<<500 S/cm) and could not reach a thermal conductivity of 1,000 W/mK(typically <<300 W/mK).

Examples 10 Heat Dissipation Systems Containing a Graphene Oxide-DerivedUnitary Graphene-Coated Graphitic Foil

We have used an infrared thermography-based hand-help device to measurethe surface temperatures of a range of microelectronic devices, such assmart phones and laptop computer. For instance, FIG. 7 shows the surfacetemperature fields of two identical smart phones running the same videoprograms for 10 minutes. One smart phone (top image) contains 2 sheetsof flexible graphite (FG) foils between the CPU and the casing, showingan external surface temperature as high as 38.6° C. The internaltemperature near the CPU is presumably much higher than 60 or 70° C., adangerous temperature that could eventually damage the device. Incontrast, the other smart phone (bottom image) contains one sheet ofGO-derived unitary graphene-coated FG foil, showing an external surfacetemperature of 25.4° C. This example has vividly demonstrated theeffectiveness of implementing a GO-coated graphitic foil-basedheat-spreader layer in a thermal management system.

As indicated in FIGS. 8(a) and 8(b), the presently invented unitarygraphene-coated graphitic foils do not have to go through anultra-high-temperature graphitization treatment. Graphitization of acarbonized resin (e.g. polyimide) or other carbon materials requires atemperature typically higher than 2,000° C., most typically higher than2,500° C. The graphitization temperature is most typically in the rangeof 2,800-3,200° C. in order for carbonized materials or pyrolyticgraphite to achieve a thermal conductivity of 1,600-1,700 W/mK. Incontrast, the typical heat treatment temperature (re-graphitizationtreatment) of the presently invented GO-coated laminates issignificantly lower than 2,500° C. and more typically lower than 1,500°(can be as low as 100-150° C.).

For instance, polyimide (PI), if carbonized and graphitized for 5 hours(including 4 hours for carbonization at 1,000-1,500° C. and 1 hour forgraphitization at 2,000° C.), exhibits a thermal conductivity of 820W/mK. In contrast, we were able to reach a thermal conductivity of 876W/mK with a heat treatment of GO at 500° C. for a total of two hours.This is very surprising and no one has ever thought that such a lowgraphitization temperature was possible. Further, a heat treatment ofthe GO-derived unitary graphene-coated graphitic foil at the same 2,000°C. for 1 hour imparts a thermal conductivity of 1,680 W/mK. Clearly,this is a dramatically faster, less energy-intensive, and morecost-effective process. The resulting products are also far superior topyrolytic graphite. The unitary graphene-coated graphitic foil, theunitary graphene layer itself (from GO gel), and the pyrolytic graphiteare three fundamentally different and patently distinct classes ofmaterials in terms of chemical composition, morphology, structure,process of production, and various properties.

In conclusion, we have successfully developed an absolutely new, novel,unexpected, and patently distinct class of highly conducting material:graphene oxide gel-derived unitary graphene layer or graphene singlecrystal. The chemical composition, structure, morphology, process ofproduction, and properties of this new class of materials arefundamentally different and patently distinct from flexible graphitefoil, polymer-derived pyrolytic graphite, CVD-derived PG (includingHOPG), and catalytic CVD graphene thin film. The thermal conductivity,electrical conductivity, scratch resistance, surface hardness, andtensile strength exhibited by the presently invented materials are muchhigher than what prior art flexible graphite sheets, graphene paper, orother graphitic films could possibly achieve. These GO-derived unitarygraphene materials have the best combination of excellent electricalconductivity, thermal conductivity, mechanical strength, surface scratchresistance, hardness, and no tendency to flake off.

We claim:
 1. A unitary graphene oxide layer or graphene oxide singlecrystal containing closely packed and chemically bonded parallelgraphene oxide planes having an inter plane spacing of 0.335 to 0.40 nm,a thickness greater than 10 nm, an electrical conductivity greater than1,500 S/cm, a thermal conductivity greater than 600 W/mK, and a physicaldensity greater than 1.8 g/cm3, and an oxygen content of 0.01% to 10% byweight, which unitary graphene oxide layer or graphene oxide singlecrystal is obtained from heat-treating a graphene oxide gel at atemperature higher than 100° C., wherein an average mis-orientationangle between two graphene oxide planes is less than 10 degrees.
 2. Theunitary graphene oxide layer or graphene oxide single crystal of claim1, containing no discrete graphite flake or graphene platelet dispersedtherein.
 3. The unitary graphene oxide layer or graphene oxide singlecrystal of claim 1, containing no complete grain boundary therein. 4.The unitary oxide graphene layer or graphene oxide single crystal ofclaim 1, wherein said graphene oxide gel is produced from particles of anatural graphite or artificial graphite composed of graphitecrystallites having an initial length L_(a) in the crystallographica-axis direction, an initial width L_(b) in the b-axis direction, and athickness L_(c) in the c-axis direction, and the unitary graphene layeror graphene single crystal has a length or width greater than theinitial L_(a) and L_(b) of the graphite crystallites.
 5. The unitarygraphene oxide layer or graphene oxide single crystal of claim 1,wherein said graphene oxide gel is produced from particles of a naturalgraphite or artificial graphite composed of graphite crystallites havingan initial length L_(a) in the crystallographic a-axis direction, aninitial width L_(b) in the b-axis direction, and a thickness L_(c) inthe c-axis direction and the unitary graphene layer or graphene singlecrystal has a length or width at least greater than twice the initialL_(a) or twice the initial L_(b) of the graphite crystallites.
 6. Theunitary graphene oxide layer or graphene oxide single crystal of claim1, wherein the chemically bonded parallel graphene planes contain acombination of sp² and sp³ electronic configurations.
 7. The unitarygraphene oxide layer or graphene oxide single crystal of claim 1,wherein said unitary graphene layer or graphene single crystal has alength or width no less than 100 μm.
 8. The unitary graphene oxide layeror graphene oxide single crystal of claim 1, wherein said unitarygraphene oxide layer or graphene oxide single crystal has a length orwidth no less than 1 mm.
 9. The unitary graphene oxide layer or grapheneoxide single crystal of claim 1, wherein said unitary graphene oxidelayer or graphene oxide single crystal has a length or width no lessthan 1 cm.
 10. The unitary graphene oxide layer or graphene oxide singlecrystal of claim 1, wherein the heat-treating temperature is from 100°C. to 1,000° C. and said unitary graphene oxide layer or graphene oxidesingle crystal has a thermal conductivity greater than 600 W/mK orelectrical conductivity greater than 2,000 S/cm.
 11. The unitarygraphene oxide layer or graphene oxide single crystal of claim 1,wherein the heat-treating temperature is from 1,000° C. to 1,500° C. andsaid unitary graphene oxide layer or graphene oxide single crystal has athermal conductivity greater than 1,300 W/mK or electrical conductivitygreater than 3,000 S/cm.
 12. The unitary graphene oxide layer orgraphene oxide single crystal of claim 1, wherein the heat-treatingtemperature is from 1500° C. to 2,500° C. and said unitary grapheneoxide layer or graphene oxide single crystal has a thermal conductivitygreater than 1,600 W /mK or electrical conductivity greater than5,000S/cm.
 13. The unitary graphene oxide layer or graphene oxide singlecrystal of claim 1, wherein the heat-treating temperature is from 2,500°C. to 3,250° C. and said unitary graphene oxide layer or graphene oxidesingle crystal has a thermal conductivity greater than 1,700 W/mK orelectrical conductivity greater than 10,000 S/cm.
 14. The unitarygraphene oxide layer or graphene oxide single crystal of claim 1, havinga thickness greater than 1 μm.
 15. The unitary graphene oxide layer orgraphene oxide single crystal of claim 1, which is optically opaque. 16.The unitary graphene oxide layer or graphene oxide single crystal ofclaim 1, having a thickness greater than 10 μm.
 17. The unitary grapheneoxide layer or graphene oxide single crystal of claim 1, having anoxygen content from 0.01% to 5% by weight.
 18. The unitary grapheneoxide layer or graphene oxide single crystal of claim 1, wherein saidgraphene oxide gel is composed of graphene oxide molecules dispersed inan acidic medium having a pH value of no higher than 5 and said grapheneoxide molecules have an oxygen content no less than 20% by weight whilein a gel state.
 19. The unitary graphene oxide layer or graphene oxidesingle crystal of claim 1, wherein said graphene oxide gel is obtainedby immersing a graphitic material in a powder or fibrous form in anoxidizing liquid medium in a reaction vessel at a reaction temperaturefor a length of time sufficient to obtain a graphene oxide gel composedof graphene oxide molecules dispersed in the liquid medium and saidgraphene oxide molecules have an oxygen content no less than 20% byweight and a molecular weight less than 43,000 g/mole while in a gelstate.
 20. The unitary graphene oxide layer or graphene oxide singlecrystal of claim 18, wherein said graphene oxide molecules have amolecular weight less than 4,000 g/mole while in a gel state.
 21. Theunitary graphene oxide layer or graphene oxide single crystal of claim18, wherein said graphene oxide molecules have a molecular weightbetween 200 g/mole and 4,000 g/mole while in a gel state.
 22. Theunitary graphene oxide layer or graphene oxide single crystal of claim1, which is produced by depositing a layer of graphene oxide gel onto asubstrate surface and by removing a residual liquid from said layer ofdeposited graphene oxide gel.
 23. The unitary graphene oxide layer orgraphene single crystal of claim 22, wherein said layer of depositedgraphene oxide gel is subjected to a heat treatment temperature of atleast 150° C. for thermal reduction and/or re-graphitization.
 24. Theunitary graphene oxide layer or graphene oxide single crystal of claim22, wherein said layer of deposited graphene oxide gel is subjected to aheat treatment temperature of at least 1 500° C. for thermal reductionand/or re-graphitization.
 25. The unitary graphene oxide layer orgraphene oxide single crystal of claim 22, wherein said layer ofdeposited graphene oxide gel is subjected to a heat treatmenttemperature from 300° C. to 1,500° C. for thermal reduction and/orre-graphitization.
 26. The unitary graphene oxide layer or grapheneoxide single crystal of claim 22, wherein said layer of depositedgraphene oxide gel is subjected to a heat treatment temperature from1,500° C. to 2,500° C. for re-graphitization.
 27. The unitary grapheneoxide layer or graphene oxide single crystal of claim 22, wherein saidlayer of deposited graphene oxide gel is subjected to a heat treatmenttemperature greater than 2,500° C.
 28. The unitary graphene oxide layeror graphene oxide single crystal of claim 1, wherein said graphene oxidegel is produced from oxidation of a graphitic material selected fromnatural graphite, artificial graphite, meso-phase carbon, meso-phasepitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbonfiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.29. The unitary graphene oxide layer or graphene oxide single crystal ofclaim 1, having a Rockwell hardness value greater than
 60. 30. Theunitary graphene oxide layer or graphene oxide single crystal of claim1, having an electrical conductivity greater than 1,500 S/cm, a thermalconductivity greater than 600 W/mK, a physical density greater than 1.8g/cm³, and/or a tensile strength greater than 40 MPa.
 31. The unitarygraphene oxide layer or graphene oxide single crystal of claim 1, havingan electrical conductivity greater than 3,000 S/cm, a thermalconductivity greater than 1,000 W/mK, a physical density greater than2.0 g/cm³, and/or a tensile strength greater than 80 MPa.
 32. Theunitary graphene oxide layer or graphene oxide single crystal of claim1, having an electrical conductivity greater than 5,000 S/cm, a thermalconductivity greater than 1,500 W/mK, a physical density greater than2.1 g/cm³ and/or a tensile strength greater than 100 MPa.